[Flutter Android]: Use proguard rules for debug builds as well

[buenaflor]

Description

If debug builds have minify enabled on Android it will interfere with our JNI usage.

we should do it similarly to Sentry Android: https://github.com/getsentry/sentry-java/blob/main/sentry-android-core/build.gradle.kts#L33

[linear]

DART-255