Enable shrink resources on android release builds for resource optimization. (#1864)

See the following page of the Android documentation about app optimization
https://developer.android.com/topic/performance/app-optimization/enable-app-optimization
for more information.

android/app/build.gradle

@@ -76,6 +76,7 @@ android {
     buildTypes {
         release {
             minifyEnabled true
+            shrinkResources true
             proguardFiles(
               getDefaultProguardFile('proguard-android-optimize.txt'),
               'proguard-rules.pro'