Clear identity when releasing display resources.

We already do this in `close()` but not in `onVirtualDisplayRemoved()`.
This causes a security exception due to missing `WAKE_LOCK` permission
on builds that use HSUM.

Fix: 286185979
Test: presubmit
Change-Id: Iebbb04f6223d465beb1fbfe0f28d3a96e4304db3
Merged-In: I40447a461409fb523be3932d67b1fc6f0843e9e9