Stop active callbacks from being stopped on starting new projection session

Ensuring that previous mediaprojection callbacks are stopped before new callbacks are registered, so new projections aren't immediately stopped after starting.

Bug: 279417791
Test: atest FrameworksServicesTests:MediaProjectionManagerServiceTest#testCreateProjection_priorProjectionGrant --iterations=20
Change-Id: Ia8f722bec5e63d7417ed93e2e56c0fccfe8e35a6