add proguard rules for release build

example/android/app/proguard-rules.pro

@@ -0,0 +1,9 @@
+-keep class io.flutter.app.** { *; }
+-keep class io.flutter.plugin.**  { *; }
+-keep class io.flutter.util.**  { *; }
+-keep class io.flutter.view.**  { *; }
+-keep class io.flutter.**  { *; }
+-keep class io.flutter.plugins.**  { *; }
+-dontwarn java.awt.*
+-keep class com.sun.jna.* { *; }
+-keepclassmembers class * extends com.sun.jna.* { public *; }