[android] Default release rollout fraction to 20% of users

Signed-off-by: Konstantin Pastbin <konstantin.pastbin@gmail.com>

android/app/build.gradle

@@ -377,6 +377,7 @@ play {
   track.set('production')
   defaultToAppBundles.set(true)
   releaseStatus.set(ReleaseStatus.IN_PROGRESS)
+  userFraction.set(0.2) // Rollout to 20% of users
   serviceAccountCredentials.set(file('google-play.json'))
 }