Add use_minimal_keep_rules flag for aapt2 minimal proguard keep rules

[Dolfik1]

Problem

aapt2 generates broad resource keep rules by default. Bazel and Gradle produce different rules for the same target:

# Bazel (current)
-keep class androidx.appcompat.widget.AlertDialogLayout { <init>(...); }

# Gradle
-keep class androidx.appcompat.widget.AlertDialogLayout { <init>(android.content.Context, android.util.AttributeSet); }

Gradle passes --proguard-minimal-keep-rules to aapt2, emitting precise member signatures. Bazel does not, so keep rules (and shrinking output) diverge.

Change

Add opt-in flag use_minimal_keep_rules. When enabled, aapt2 link receives --proguard-minimal-keep-rules, matching Gradle.

Off by default. Enable with:

build --define=use_minimal_keep_rules=true

Mirrors the existing conditionalKeepRules / --proguard-conditional-keep-rules path.

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