Add support for AndroidX fragments

Summary:
Fix #931

This is not how I would *like* to fix this, but it should do the job.
When the switch over to AndroidX was made, the overall abstraction
started to leak and we really need to remodel this in its entirety.
There's also the question of whether we want to support both support
fragments and AndroidX fragments or not. Right now it's kinda-sorta
supported but only under some circumstances, which is not great.

Test Plan:
Changed the sample app to include some AndroidX fragments and they
now show up (again) in the view hierarchy:

android/src/main/java/com/facebook/flipper/plugins/inspector/descriptors/ActivityDescriptor.java

@@ -8,6 +8,7 @@
 package com.facebook.flipper.plugins.inspector.descriptors;
 
 import android.app.Activity;
+import android.util.Log;
 import android.view.Window;
 import com.facebook.flipper.core.FlipperDynamic;
 import com.facebook.flipper.core.FlipperObject;
@@ -25,6 +26,8 @@
 
 public class ActivityDescriptor extends NodeDescriptor<Activity> {
 
+  private static final String TAG = "ActivityDescriptor";
+
   @Override
   public void init(Activity node) {}
 
@@ -112,9 +115,14 @@ private static List<Object> getDialogFragments(FragmentCompat compat, Activity a
     }
 
     FragmentManagerAccessor fragmentManagerAccessor = compat.forFragmentManager();
-    List<Object> addedFragments = fragmentManagerAccessor.getAddedFragments(fragmentManager);
+    List<Object> addedFragments = null;
+    try {
+      addedFragments = fragmentManagerAccessor.getAddedFragments(fragmentManager);
+    } catch (Exception e) {
+      Log.e(TAG, "Failed to obtain list of fragments.", e);
+    }
     if (addedFragments == null) {
-      return Collections.EMPTY_LIST;
+      return Collections.emptyList();
     }
 
     final List<Object> dialogFragments = new ArrayList<>();

android/src/main/java/com/facebook/flipper/plugins/inspector/descriptors/utils/stethocopies/FragmentCompat.java

@@ -8,8 +8,14 @@
 package com.facebook.flipper.plugins.inspector.descriptors.utils.stethocopies;
 
 import android.app.Activity;
+import android.content.Context;
 import android.os.Build;
+import android.util.Log;
+
+import androidx.fragment.app.FragmentManager;
+
 import java.lang.reflect.Field;
+import java.util.Collections;
 import java.util.List;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.NotThreadSafe;
@@ -37,7 +43,7 @@ public abstract class FragmentCompat<
 
   static {
     sHasSupportFragment =
-        ReflectionUtil.tryGetClassForName("android.support.v4.app.Fragment") != null;
+        ReflectionUtil.tryGetClassForName("androidx.fragment.app.Fragment") != null;
   }
 
   @Nullable
@@ -82,20 +88,22 @@ static class FragmentManagerAccessorViaReflection<FRAGMENT_MANAGER, FRAGMENT>
     @Nullable
     @Override
     public List<FRAGMENT> getAddedFragments(FRAGMENT_MANAGER fragmentManager) {
-      // This field is actually sitting on FragmentManagerImpl, which derives from FragmentManager.
-      if (mFieldMAdded == null) {
-        Field fieldMAdded =
-            ReflectionUtil.tryGetDeclaredField(fragmentManager.getClass(), "mAdded");
-
-        if (fieldMAdded != null) {
-          fieldMAdded.setAccessible(true);
-          mFieldMAdded = fieldMAdded;
+      if (fragmentManager instanceof android.app.FragmentManager) {
+        // This field is actually sitting on FragmentManagerImpl, which derives from
+        // FragmentManager.
+        if (mFieldMAdded == null) {
+          Field fieldMAdded =
+              ReflectionUtil.tryGetDeclaredField(fragmentManager.getClass(), "mAdded");
+
+          if (fieldMAdded != null) {
+            fieldMAdded.setAccessible(true);
+            mFieldMAdded = fieldMAdded;
+          }
         }
+      } else if (fragmentManager instanceof androidx.fragment.app.FragmentManager) {
+        return (List<FRAGMENT>) ((FragmentManager) fragmentManager).getFragments();
       }
-
-      return (mFieldMAdded != null)
-          ? (List<FRAGMENT>) ReflectionUtil.getFieldValue(mFieldMAdded, fragmentManager)
-          : null;
+      return Collections.emptyList();
     }
   }
 }

android/src/main/java/com/facebook/flipper/plugins/inspector/descriptors/utils/stethocopies/FragmentCompatUtil.java

@@ -8,11 +8,16 @@
 package com.facebook.flipper.plugins.inspector.descriptors.utils.stethocopies;
 
 import android.app.Activity;
+import android.util.Log;
 import android.view.View;
+
+import java.util.Collections;
 import java.util.List;
 import javax.annotation.Nullable;
 
 public final class FragmentCompatUtil {
+  private static final String TAG = "FragmentCompatUtil";
+
   private FragmentCompatUtil() {}
 
   public static boolean isDialogFragment(Object fragment) {
@@ -79,7 +84,13 @@ private static Object findFragmentForViewInActivity(
   @Nullable
   private static Object findFragmentForViewInFragmentManager(
       FragmentCompat compat, Object fragmentManager, View view) {
-    List<?> fragments = compat.forFragmentManager().getAddedFragments(fragmentManager);
+    List<?> fragments;
+    try {
+      fragments = compat.forFragmentManager().getAddedFragments(fragmentManager);
+    } catch (Exception e) {
+      fragments = Collections.emptyList();
+      Log.e(TAG, "Failed to obtain list of fragments.", e);
+    }
 
     if (fragments != null) {
       for (int i = 0, N = fragments.size(); i < N; ++i) {