Allow resource override on max number of actions in PiP

Moved the query of max number of pip actions from
ActivityTaskManagerService to ActivityTaskManager.

Note that TaskInfo#pictureInPictureParams is exposed via
TaskInfo#getPictureInPictureParams (annotated as @TestApi).

Bug: 168800594
Test: atest PinnedStackTests#testMaxNumberOfActions \
            PinnedStackTests#testFillMaxAllowedActions \
            PinnedStackTests#testRejectExceededActions
Change-Id: Ibcf2302e3ec87909543a1d6da0b64fd4565a061c