build: configure release signing for Android APK

Reads keystore credentials from keystore.properties (gitignored).
Keystore file (myice.keystore) is also gitignored.

.gitignore

@@ -6,5 +6,7 @@ local.properties
 .DS_Store
 *.apk
 *.aab
+*.keystore
+keystore.properties
 *.xcworkspace/xcuserdata/
 iosApp/iosApp.xcodeproj/xcuserdata/