Add default minSdkVersion

This avoids some not needed permissions to be automatically included by Gradle during the Manifest merge phase.
It should be harmless otherwise, considering the Flutter's minSdkVersion is 16 anyways.

android/build.gradle

@@ -26,6 +26,7 @@ android {
 
     defaultConfig {
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
+        minSdkVersion 16
     }
     lintOptions {
         disable 'InvalidPackage'