Fix problem with starting a translucent activity in onCreate().

Fixes issue #2437252: Starting activity by means of startActivityForResult
causes 5 seconds delay if "android:windowIsTranslucent" is true

The optimization to avoid showing an activity window when a new
activity is being started was a little too aggressive.  Now it
avoids doing this if there is not actually a fullscreen activity
on top to cover it.

Change-Id: I630e37a1f1d3b874b5a25572cbf887cebc2e3e91