Fix creating of app theme snapshots

Apparently locking a HW bitmap makes it super slow to draw.
Instead, draw it into a bitmap, copy it into a hw bitmap, and then
extract the GraphicBuffer.

Test: Open Camera, go home, reopen camera (and maybe go to recents)
Change-Id: Id2371dcdad95f07a680e6c56c0403b3ad815a3f3
Fixes: 38510859