Wait for target package MessageQueue empty before continuing

By using Looper.myQueue().addIdleHandler(), the variance for perf tests
decreases dramatically.

Before:
registeredBroadcast_mean=12162588
registeredBroadcast_median=9496511
registeredBroadcast_stddev=10821157
manifestBroadcastRunning_mean=16673251
manifestBroadcastRunning_median=11859325
manifestBroadcastRunning_stddev=13329366
manifestBroadcastNotRunning_mean=59187569
manifestBroadcastNotRunning_median=55761932
manifestBroadcastNotRunning_stddev=14805616

Now:
registeredBroadcast_mean=940791
registeredBroadcast_median=889583
registeredBroadcast_stddev=157170
manifestBroadcastRunning_mean=1396281
manifestBroadcastRunning_median=1364531
manifestBroadcastRunning_stddev=189226
manifestBroadcastNotRunning_mean=59161472
manifestBroadcastNotRunning_median=56538781
manifestBroadcastNotRunning_stddev=14414728

Test: m ActivityManagerPerfTestsTestApp ActivityManagerPerfTests
Test: adb install \
$OUT/data/app/ActivityManagerPerfTestsTestApp/ActivityManagerPerfTestsTestApp.apk
Test: adb install \
$OUT/data/app/ActivityManagerPerfTests/ActivityManagerPerfTests.apk
Test: adb shell am instrument -w
com.android.frameworks.perftests.amtests/android.support.test.runner.AndroidJUnitRunner

Change-Id: I05714294c53f3d013a080dfaa617e1fc92fd14e0