[Handoff][5/N] Clear Calling Identity when Requesting Handoff

`TaskContinuityManager` needs permissions that the caller may not have when requesting Handoff, so clear the calling identity during a Handoff request.

Test: Validated manually. CTS tests in the future will fully test this
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: I7e42ab88999e5fc2745724f997c77c265395d4a2