Ensure clearCallingIdentity for getDisplayContentOrCreate

If a new display is created, it may acquire wakelock for freezing
display when configuring the display. That may raise an exception
that the calling uid is not the same as the owner uid.

Fix: 147778342
Test: atest DisplayPolicyTests
Change-Id: Ib31c0d1f98964874a0ab97725d43f6bbd009062c