Refine system service proguard rules

Refine proguard rules for system services to prepare for rolling out
full system server R8 optimization.

Added proguard rules to keep auto-generated Java aconfig flags to
prevent Flag function from being inlined / removed since R8 do not have
context on flags that are referenced outside of the module.

Bug: 313546597
Change-Id: I184cd7a79acc7fa2d9db66438df1f0b50d951fd9
Test: Local build and boot with full system server optimization

services/proguard.flags

@@ -54,7 +54,10 @@
 -keep,allowoptimization,allowaccessmodification class android.app.admin.flags.FeatureFlagsImpl { *; }
 -keep,allowoptimization,allowaccessmodification class com.android.server.input.NativeInputManagerService$NativeImpl { *; }
 -keep,allowoptimization,allowaccessmodification class com.android.server.ThreadPriorityBooster { *; }
--keep,allowaccessmodification class android.app.admin.flags.Flags { *; }
+
+# Keep all aconfig Flag class as they might be statically referenced by other packages
+# An merge or inlining could lead to missing dependencies that cause run time errors
+-keepclassmembernames class android.**.Flags, com.android.**.Flags { public *; }
 
 # Referenced via CarServiceHelperService in car-frameworks-service (avoid removing)
 -keep public class com.android.server.utils.Slogf { *; }