Add support for explicit Auto Pip

Adds a new parameter to PictureInPictureParams that
allows activities to explicitly allow the system to
put them in PIP mode without them having to call
enterPictureInPictureMode().

Test: atest PinnedStackTests#testAutoPipAllowedBypassesExplicitEnterPip
Bug: 163048957
Change-Id: Ib6fda0e231408f2756a4e1b9fda08536889a1be5