Keep ArtStasLog write() functions

Some of these functions are called from IsolatedCompilationMetrics, a
class added to the system server classpath by the (non-updatabale)
com.android.compos APEX. Without this they are stripped as apparently
unused, leading to system server crashing when we try to call them.

Bug: 231271783
Test: Stage APEX, observe no crash, job scheduled successfully
Test: atest CompOsSigningHostTest#verifyArtUpgradeGeneratesAnyArtifacts
Change-Id: I5ad0249b0aae87e98ebdd3deb6e8c58bfb691ebc

services/proguard.flags

@@ -33,6 +33,11 @@
   public <init>(...);
 }
 
+# Accessed from com.android.compos APEX
+-keep,allowoptimization,allowaccessmodification class com.android.internal.art.ArtStatsLog {
+   public static void write(...);
+}
+
 # Binder interfaces
 -keep,allowoptimization,allowaccessmodification class * extends android.os.IInterface
 -keep,allowoptimization,allowaccessmodification class * extends android.os.IHwInterface