Make sure screen is turned on when checking for activity to be ready

If the screen is off, the test activity will be paused and the tests will
fail when asserting that the activity is ready

Fix: 305917812
Test: WallpaperManagerTest#testSharedWallpaperVisibilityBehindActivity_onLockScreen
WallpaperManagerTest#testIndependentWallpaperVisibilityBehindActivity_onLockScreen
starting the tests with the screen off

Change-Id: I0f9892d9da090be9f23a4eea4f6af5724b00e532