Support package signature faking used by µg GmsCore

Unfortunately due to GMSCore libraries for clients carrying out signature checks on the GMS app,
users are not able to use open source equivalents, and are being strong-armed (by apps) into using
non-free libraries for these functions. NoGapps is an open source implementation of these libraries.

Unfortunately, a patch like this is necessary in order to permit apps to be convinced they are talking
to the closed-source GMSCore. As this "DRM" is implemented at app-level, it's possible to do this,
without the Android OS itself being "fooled" by the faked signatures.

This commit is from the NoGapps project, I am just submitting it on their behalf.

See https://github.com/microg/android_packages_apps_GmsCore
Explanation: https://gerrit.omnirom.org/#/c/8672/

Change-Id: I61a82bcb5d32fe3c0c71a74d2c3db24262d0556b

core/java/android/content/pm/PackageParser.java

@@ -609,10 +609,23 @@ public static PackageInfo generatePackageInfo(PackageParser.Package p,
             }
         }
         if ((flags&PackageManager.GET_SIGNATURES) != 0) {
-           int N = (p.mSignatures != null) ? p.mSignatures.length : 0;
-           if (N > 0) {
-                pi.signatures = new Signature[N];
-                System.arraycopy(p.mSignatures, 0, pi.signatures, 0, N);
+            boolean handledFakeSignature = false;
+            try {
+                if (p.requestedPermissions.contains("android.permission.FAKE_PACKAGE_SIGNATURE") && p.mAppMetaData != null
+                        && p.mAppMetaData.get("fake-signature") instanceof String) {
+                    pi.signatures = new Signature[] {new Signature(p.mAppMetaData.getString("fake-signature"))};
+                    handledFakeSignature = true;
+                }
+            } catch (Throwable t) {
+                // We should never die because of any failures, this is system code!
+                Log.w("PackageParser.FAKE_PACKAGE_SIGNATURE", t);
+            }
+            if (!handledFakeSignature) {
+                int N = (p.mSignatures != null) ? p.mSignatures.length : 0;
+                if (N > 0) {
+                    pi.signatures = new Signature[N];
+                    System.arraycopy(p.mSignatures, 0, pi.signatures, 0, N);
+                }
             }
         }
         return pi;

core/res/AndroidManifest.xml

@@ -1825,6 +1825,13 @@
         android:label="@string/permlab_getPackageSize"
         android:description="@string/permdesc_getPackageSize" />
 
+    <!-- Allows an application to change the package signature as seen by applications -->
+    <permission android:name="android.permission.FAKE_PACKAGE_SIGNATURE"
+                android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
+                android:protectionLevel="dangerous"
+                android:label="@string/permlab_fakePackageSignature"
+                android:description="@string/permdesc_fakePackageSignature" />
+
     <!-- @deprecated No longer useful, see
          {@link android.content.pm.PackageManager#addPackageToPreferred}
          for details. -->

core/res/res/values/strings.xml

@@ -1285,6 +1285,11 @@
     <!-- Description of an application permission, listed so the user can choose whether they want to allow the application to do this. -->
     <string name="permdesc_getPackageSize">Allows the app to retrieve its code, data, and cache sizes</string>
 
+    <!-- Title of an application permission, listed so the user can choose whether they want to allow the application to do this. -->
+    <string name="permlab_fakePackageSignature">mimic package signature</string>
+    <!-- Description of an application permission, listed so the user can choose whether they want to allow the application to do this. -->
+    <string name="permdesc_fakePackageSignature">Allows the app to use mimic another app\'s package signature.</string>
+
     <!-- Title of an application permission, listed so the user can choose whether they want to allow the application to do this. -->
     <string name="permlab_installPackages">directly install apps</string>
     <!-- Description of an application permission, listed so the user can choose whether they want to allow the application to do this. -->