frameworks/base: Fix the random UI corruption

On low memory device, when a view goes to background. Its HW layer cache
will be destroyed to release memory. There is a very rare case that the
hardware render is not valid when destroy the layer, so the layer is not
destroyed and reused when the view comes to foreground. But the GL
context has changed, so the view is not rendered correctly due to its
HW layer cache. To fix this issue, destroy the layer in a safelyRun. Make
sure the layer is destroyed correctly.

Change-Id: I1c714e1d53db7353d5169e874a7cd0f527c3f67b
CRs-Fixed: 623239
Do not show Home behind full screen activity

When ensureActivitiesVisibleLocked goes through foreground activity
stack and reaches non-fullscreen activity, it sets showHomeBehindStack
variable to true.

If there is a fullscreen activity behind, showHomeBehindStack remains
unchanged, which causes Home application to be displayed anyway.
In this case user will see a fullscreen activity and Home activity
simultaneously.

To fix the issue we set showHomeBehindStack to false when we reach
fullscreen activity in the activity stack.

This was made visible by the following commit:
446ef1de8d373c1b017df8d19ebf9a47811fb402

Change-Id: I535c1283a4e26f5cf606375b837d4b7195324af0