Limit JIT tier for javac

javac is generally a short-lived JVM invocation that doesn't greatly
benefit from deep JIT tiering. As such, favor faster startup time
by limiting to tier 1 JIT for javac execution. This should greatly
reduce overall CPU usage from Java compilation for an arbitrary
Android build (~20%), and deliver a small but modest improvement in
overall Java build times (~5%).

Representative incremental build times with this change:
  * real 18m3.507s
  * user 563m4.838s
and without:
  * real 19m11.909s
  * user 751m27.790s

Follow-up changes will apply similar flags for other short-lived JVM
build commands.

Bug: 205303544
Test: m
Change-Id: Ic6315b976063f4330902930a6cf5e5cb394c554c

java/config/config.go

@@ -60,7 +60,7 @@ var (
 
 const (
 	JavaVmFlags  = `-XX:OnError="cat hs_err_pid%p.log" -XX:CICompilerCount=6 -XX:+UseDynamicNumberOfGCThreads`
-	JavacVmFlags = `-J-XX:OnError="cat hs_err_pid%p.log" -J-XX:CICompilerCount=6 -J-XX:+UseDynamicNumberOfGCThreads`
+	JavacVmFlags = `-J-XX:OnError="cat hs_err_pid%p.log" -J-XX:CICompilerCount=6 -J-XX:+UseDynamicNumberOfGCThreads -J-XX:+TieredCompilation -J-XX:TieredStopAtLevel=1`
 )
 
 func init() {