Initial implementation of snapshots

All this functionality is hidden behind a flag. If this flag is
active, we disable the regular screenshots.

Instead, we take a screenshot when an app transition for which a
task is disappearing is starting. The screenshot gets stored
into a gralloc buffer. SystemUI uses a new method to retrieve
a snapshot gralloc buffer and then draws it using GraphicBuffer.
createHardwareBitmap().

When starting an existing activity in an existing tasks, or when
bringing an existing tasks to front from recents, we add a new
snapshot starting window. For that, we reuse the existing
starting window, but when creating the window, we use a fake
window that draws the contents of the starting window.

Test: runtest frameworks-services -c
com.android.server.wm.TaskSnapshotControllerTest
Bug: 31339431
Change-Id: If72df07b3e56f30413db5029d0887b8c9665aaf4

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