ActivityManager: Donot take launcher screenshot.

Currently ActivityManger takes screenshot for all activities
on an activity switch, for maintaining the screenshots in recent
app list. This can be skipped for launcher, as launcher is not
present in the recent app list.

Change-Id: I22f7cda2c63483ab50e0df4428151503892b7d9b
Signed-off-by: iceandfire <arhamjamal@gmail.com>

Conflicts:
	services/java/com/android/server/am/ActivityStack.java