GlobalScreenshot - Fix race condition

It is possible for the call to system windows to occur slightly after
the ChooserActivity gets launched, therefore preventing it from ever
showing. Wait on the future before attempting to launch the activity.

Bug: 134976976
Test: Screenshot, then share over and over
Change-Id: Iba98a079412f8e35643f3a1db04e749b3c654637
2 files changed