Fix the binder calling identity in setVrMode

When an app calls setVrMode, we end up moving the task stack which needs
MANAGE_ACTIVITY_STACKS permission which the app can never get. So, clear
the binder identity of the app while making this call.

Testing: android.vr.cts.VrSetFIFOThreadTest
Bug: 65743448

Change-Id: Id37968f4fb5531edadb0726e5fc04b2470d2d0e5
Signed-off-by: Karthik Ravi Shankar <karthikrs@google.com>
(cherry picked from commit ac214bbfa2ebd8863cb36e3f002760ef3293b0da)