Use proguard in release builds to shink size

Author: sastred

android/app/build.gradle

@@ -74,7 +74,7 @@ def enableSeparateBuildPerCPUArchitecture = false
 /**
  * Run Proguard to shrink the Java bytecode in release builds.
  */
-def enableProguardInReleaseBuilds = false
+def enableProguardInReleaseBuilds = true
 
 android {
     compileSdkVersion 23