test to enable predictive back gestures

ref #445

Author: dreautall

android/app/src/main/AndroidManifest.xml

@@ -3,7 +3,7 @@
             android:label="Waterfly III"
             android:name="${applicationName}"
             android:icon="@mipmap/ic_launcher"
-            android:enableOnBackInvokedCallback="false"
+            android:enableOnBackInvokedCallback="true"
             android:usesCleartextTraffic="true"
             android:networkSecurityConfig="@xml/network_security_config"
     >

lib/app.dart

@@ -268,6 +268,12 @@ class _WaterflyAppState extends State<WaterflyApp> {
                 appBarTheme: const AppBarTheme(
                   shape: RoundedRectangleBorder(),
                 ),
+                pageTransitionsTheme: const PageTransitionsTheme(
+                  builders: <TargetPlatform, PageTransitionsBuilder>{
+                    TargetPlatform.android:
+                        PredictiveBackPageTransitionsBuilder(),
+                  },
+                ),
               ),
               darkTheme: ThemeData(
                 brightness: Brightness.dark,