Reverts flutter/flutter#136880
Initiated by: camsim99
This change reverts the following previous change:
Original Description:
Since the original PR that supposedly enabled proguard, it was using the android proguard rules that disable optimizations. See initial PR in [0]

This PR changes the flutter gradle plugin to use the `proguard-android-optimize.txt` (instead of `proguard-android.txt`) which will enable optimizations/shrinking of platform code (i.e. java/kotlin).

For a simple flutter hello world this results in a 25% reduction in the resulting DEX file (`classes.dex` of the APK).

[0] f098de1f

Fixes https://github.com/flutter/flutter/issues/136879