Fix potential crash due to optional Google Play Services dependency

GoogleApiAvailability will return true when any Google Play Services SDK
is included as a dependency and Google Play Services is available on the
device. However the Wallet component may not be present and would cause
a crash.

Braintree/src/main/java/com/braintreepayments/api/models/AndroidPayConfiguration.java

@@ -7,6 +7,7 @@
 import com.braintreepayments.api.interfaces.BraintreeResponseListener;
 import com.google.android.gms.common.ConnectionResult;
 import com.google.android.gms.common.GoogleApiAvailability;
+import com.google.android.gms.wallet.Wallet;
 
 import org.json.JSONArray;
 import org.json.JSONException;
@@ -71,9 +72,11 @@ public static AndroidPayConfiguration fromJson(JSONObject json) {
      */
     public boolean isEnabled(Context context) {
         try {
+            Class.forName(Wallet.class.getName());
+
             return mEnabled && GoogleApiAvailability.getInstance().isGooglePlayServicesAvailable(context) ==
                     ConnectionResult.SUCCESS;
-        } catch (NoClassDefFoundError e) {
+        } catch (ClassNotFoundException | NoClassDefFoundError e) {
             return false;
         }
     }

Braintree/src/main/java/com/braintreepayments/api/models/VisaCheckoutConfiguration.java

@@ -2,6 +2,7 @@
 
 import com.braintreepayments.api.Json;
 import com.visa.checkout.Profile.CardBrand;
+import com.visa.checkout.VisaCheckoutSdk;
 
 import org.json.JSONObject;
 
@@ -25,7 +26,7 @@ public class VisaCheckoutConfiguration {
      */
     private static boolean isVisaCheckoutSDKAvailable() {
         try {
-            Class.forName("com.visa.checkout.VisaCheckoutSdk");
+            Class.forName(VisaCheckoutSdk.class.getName());
             return true;
         } catch (ClassNotFoundException e) {
             return false;