Optimize CPU time when running d8 and r8

Pass -XX:+TieredCompilation -XX:TieredStopAtLevel=1 to the JVM to
reduce the time spent JITing, which in local experiments reduced
the CPU time of R8 while increasing the wall time, and reduced the
total number of runnable threads.

Without the new flags:
$ time out/soong/host/linux-x86/bin/r8-compat-proguard -injars out/target/common/obj/APPS/Dialer_intermediates/classes.jar ...
real	0m30.090s
user	4m35.337s
sys	0m12.853s

With the new flags:
$ time out/soong/host/linux-x86/bin/r8-compat-proguard -JXX:+TieredCompilation -JXX:TieredStopAtLevel=1 -injars out/target/common/obj/APPS/Dialer_intermediates/classes.jar ...
real	0m35.674s
user	2m45.135s
sys	0m9.531s

The increase in wall time should be offset by the better parallelism from
the reduced CPU time.

Bug: 119270658
Test: m checkbuild
Change-Id: I8fc78b5ef1f8c6f51fd97a29461b96ae2b35cada

java/config/config.go

@@ -61,6 +61,7 @@ func init() {
 
 	pctx.StaticVariable("JavacHeapSize", "2048M")
 	pctx.StaticVariable("JavacHeapFlags", "-J-Xmx${JavacHeapSize}")
+	pctx.StaticVariable("DexFlags", "-JXX:+TieredCompilation -JXX:TieredStopAtLevel=1")
 
 	pctx.StaticVariable("CommonJdkFlags", strings.Join([]string{
 		`-Xmaxerrs 9999999`,

java/config/makevars.go

@@ -81,4 +81,6 @@ func makeVarsProvider(ctx android.MakeVarsContext) {
 
 	ctx.Strict("CLASS2GREYLIST", "${Class2Greylist}")
 	ctx.Strict("HIDDENAPI", "${HiddenAPI}")
+
+	ctx.Strict("DEX_FLAGS", "${DexFlags}")
 }

java/dex.go

@@ -25,7 +25,7 @@ import (
 var d8 = pctx.AndroidStaticRule("d8",
 	blueprint.RuleParams{
 		Command: `rm -rf "$outDir" && mkdir -p "$outDir" && ` +
-			`${config.D8Cmd} --output $outDir $d8Flags $in && ` +
+			`${config.D8Cmd} ${config.DexFlags} --output $outDir $d8Flags $in && ` +
 			`${config.SoongZipCmd} $zipFlags -o $outDir/classes.dex.jar -C $outDir -f "$outDir/classes*.dex" && ` +
 			`${config.MergeZipsCmd} -D -stripFile "**/*.class" $out $outDir/classes.dex.jar $in`,
 		CommandDeps: []string{
@@ -40,7 +40,7 @@ var r8 = pctx.AndroidStaticRule("r8",
 	blueprint.RuleParams{
 		Command: `rm -rf "$outDir" && mkdir -p "$outDir" && ` +
 			`rm -f "$outDict" && ` +
-			`${config.R8Cmd} -injars $in --output $outDir ` +
+			`${config.R8Cmd} ${config.DexFlags} -injars $in --output $outDir ` +
 			`--force-proguard-compatibility ` +
 			`--no-data-resources ` +
 			`-printmapping $outDict ` +