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).

Note for users:

For some users this may result in issues because their java/kotlin code is
now better optimized & tree shaken and thereby symbols may be no longer
available or being obfuscated.

To fix those issues it's best to craft precise proguard rules describing the
extra symbols that are needed by the app (see [1]). But it's also possible to
opt out entirely of optimizations by using the unoptimized proguard rules.

To add custom proguard rules or use the unoptimized android rules, one can
update `android/app/build.gradle`:
```
android {
    ...
    buildTypes {
        release {
            ...
+            proguardFiles(
+                // Not ideal: Disables optimizations by using unoptimized android rules.
+                getDefaultProguardFile("proguard-android.txt"),
+
+                // Better: Have precise keep rules to only keep things that are needed.
+                "custom-rules.pro",
+            )
        }
    }
}
```


[0] f098de1f
[1] https://developer.android.com/build/shrink-code

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