Revert "Revert "Use D8 by default for android_test""

This reverts commit 4e445be558e5b41dd8ac279ed6d25c41a9f0cd7b.

Reason for revert: GtsExoPlayerTestCases now uses multidex.

Test: m + manual builds for test_suites_x86_64_coverage
Change-Id: I608b3b79137b216f546446e1c4f89f4542572581
Bug: 192032291
diff --git a/java/dex.go b/java/dex.go
index 13d6e4a..c943938 100644
--- a/java/dex.go
+++ b/java/dex.go
@@ -36,8 +36,8 @@
 	Main_dex_rules []string `android:"path"`
 
 	Optimize struct {
-		// If false, disable all optimization.  Defaults to true for android_app and android_test
-		// modules, false for java_library and java_test modules.
+		// If false, disable all optimization.  Defaults to true for android_app and
+		// android_test_helper_app modules, false for android_test, java_library, and java_test modules.
 		Enabled *bool
 		// True if the module containing this has it set by default.
 		EnabledByDefault bool `blueprint:"mutated"`