Fix two ActivityManagerTests when no Home Screen is present

The following two tests where failing on ARC++ due to the fact that
they expected a Home Screen to be present. This CL fixes this expectation
by checking for a present Home Screen first.

android.app.cts.ActivityManagerTest#testTimeTrackingAPI_ChainedActivityExit
android.app.cts.ActivityManagerTest#testTimeTrackingAPI_SimpleStartExit

BUG: 33642900
TEST: run cts -m CtsAppTestCases -t android.app.cts.ActivityManagerTest#testTimeTrackingAPI_ChainedActivityExit
TEST: run cts -m CtsAppTestCases -t android.app.cts.ActivityManagerTest#testTimeTrackingAPI_SimpleStartExit
Change-Id: Iee3ba5b5d86c968e6d6276e8d1b628223074a719
1 file changed