Benchmark fix.

Previous change removed full GC from uninstall flow.
Add a shell command to force full GC.
Use the shell command to get to a clean state after install/uninstall
cycle.

==========================================================================================================================================================
(previous) Runtime.getRuntime().gc();
---------------------------------
com.android.perftests.packagemanager (1 Test(s))
[1/1] android.content.pm.PackageInstallerBenchmark#commit_threeMultiplePackageSessions_untilFinishBenchmark: PASSED (02m10.101s)
        commit_threeMultiplePackageSessions_untilFinishBenchmark_min (ns): 689576794
        commit_threeMultiplePackageSessions_untilFinishBenchmark_median (ns): 714554346
        commit_threeMultiplePackageSessions_untilFinishBenchmark_mean (ns): 711425403
        commit_threeMultiplePackageSessions_untilFinishBenchmark_standardDeviation: 17602400
---------------------------------
com.android.perftests.packagemanager (1 Test(s))
[1/1] android.content.pm.PackageInstallerBenchmark#commit_threeMultiplePackageSessions_untilFinishBenchmark: PASSED (02m24.778s)
        commit_threeMultiplePackageSessions_untilFinishBenchmark_min (ns): 733331347
        commit_threeMultiplePackageSessions_untilFinishBenchmark_median (ns): 761475101
        commit_threeMultiplePackageSessions_untilFinishBenchmark_mean (ns): 768526419
        commit_threeMultiplePackageSessions_untilFinishBenchmark_standardDeviation: 30700805
==========================================================================================================================================================
(current) VMRuntime.getRuntime().requestConcurrentGC();
---------------------------------
com.android.perftests.packagemanager (1 Test(s))
[1/1] android.content.pm.PackageInstallerBenchmark#commit_threeMultiplePackageSessions_untilFinishBenchmark: PASSED (01m42.290s)
        commit_threeMultiplePackageSessions_untilFinishBenchmark_min (ns): 781405493
        commit_threeMultiplePackageSessions_untilFinishBenchmark_median (ns): 793544987
        commit_threeMultiplePackageSessions_untilFinishBenchmark_mean (ns): 802736110
        commit_threeMultiplePackageSessions_untilFinishBenchmark_standardDeviation: 21416153
---------------------------------
com.android.perftests.packagemanager (1 Test(s))
[1/1] android.content.pm.PackageInstallerBenchmark#commit_threeMultiplePackageSessions_untilFinishBenchmark: PASSED (01m55.299s)
        commit_threeMultiplePackageSessions_untilFinishBenchmark_min (ns): 834772461
        commit_threeMultiplePackageSessions_untilFinishBenchmark_median (ns): 845870959
        commit_threeMultiplePackageSessions_untilFinishBenchmark_mean (ns): 853225375
        commit_threeMultiplePackageSessions_untilFinishBenchmark_standardDeviation: 20008646
==========================================================================================================================================================
(fixed)
---------------------------------
com.android.perftests.packagemanager (1 Test(s))
[1/1] android.content.pm.PackageInstallerBenchmark#commit_threeMultiplePackageSessions_untilFinishBenchmark: PASSED (01m53.377s)
        commit_threeMultiplePackageSessions_untilFinishBenchmark_min (ns): 692397872
        commit_threeMultiplePackageSessions_untilFinishBenchmark_median (ns): 724355473
        commit_threeMultiplePackageSessions_untilFinishBenchmark_mean (ns): 718845819
        commit_threeMultiplePackageSessions_untilFinishBenchmark_standardDeviation: 24269029
---------------------------------
com.android.perftests.packagemanager (1 Test(s))
[1/1] android.content.pm.PackageInstallerBenchmark#commit_threeMultiplePackageSessions_untilFinishBenchmark: PASSED (02m12.683s)
        commit_threeMultiplePackageSessions_untilFinishBenchmark_min (ns): 745539006
        commit_threeMultiplePackageSessions_untilFinishBenchmark_median (ns): 793251286
        commit_threeMultiplePackageSessions_untilFinishBenchmark_mean (ns): 788150394
        commit_threeMultiplePackageSessions_untilFinishBenchmark_standardDeviation: 27868293

Bug: 222961573
Fixes: 222961573
Test: atest PackageInstallerBenchmark
Change-Id: I0dda41781b0de5f3350498a7ca3a6d1296f63a04