Enable R8 full mode by default for apps

This provides significant size savings over the default compatibility
mode, roughly ~20MB in aggregate for a recent Pixel build. Targets can opt out using `proguard_compatibility = true`.

Future work will enable for all targets, not just apps, but this
incremental change yields the most benefit.

Bug: 215530220
Test: m + presubmit
Change-Id: I52a9b27ebe71835849e200e23ae2a250eaf1bbe1
diff --git a/java/dex.go b/java/dex.go
index 7e7da00..7dd14bd 100644
--- a/java/dex.go
+++ b/java/dex.go
@@ -45,8 +45,8 @@
 		// Whether to continue building even if warnings are emitted.  Defaults to true.
 		Ignore_warnings *bool
 
-		// If true, runs R8 in Proguard compatibility mode (default).
-		// Otherwise, runs R8 in full mode.
+		// If true, runs R8 in Proguard compatibility mode, otherwise runs R8 in full mode.
+		// Defaults to false for apps, true for libraries and tests.
 		Proguard_compatibility *bool
 
 		// If true, optimize for size by removing unused code.  Defaults to true for apps,