Add GameSession#takeScreenshot API

The screenshot is taken using a new method added to the
WindowManagerService: captureTaskSnapshot. The existing
WindowManagerService methods for getting a TaskSnapshot
are not suitable because they rely on previously cached
snapshots (e.g., taken when the task is put into the background).

To access the WindowManagerService functionality from
the GameSessionService, an IBinder is passed from the
GameServiceProviderInstanceImpl, which is running on the system server
side (and thus can call the new method) to the GameSessionService when
it is created. The GameSessionService then makes this reference
available to each GameSession when the GameSession is created via the
new GameSession#attach method. The GameSession can then use the IBinder
reference to make an IPC back to the GameServiceProviderInstanceImpl
instance which hosts the GameSessionService. This reference is then used
by the GameSession to request a screenshot.

By using the IBinder in this way, only GameSessions which are created
via the system GameService can access the sensitive screenshot
functionality implemented by GameServiceProviderInstanceImpl via the
new captureTaskSnapshot method.

Test: Manual e2e testing
Bug: 210119689
Bug: 202414447
Bug: 202417255
CTS-Coverage-Bug: 206128693
Change-Id: If42dc9a5a5b6068db8670666a371117cf5865f20