Fix window token leak on multi-display targets

The DisplayManager.getDisplays() API might return a different
number of displays depending on the calling UID. An inconsistent
behavior could be observed when running CTS tests, where
addWindowTokensForAllDisplays() and onRemoved() methods could
be called with either SYSTEM_UID or SHELL_UID for the same
service connection. This resulted in adding window tokens for
more displays but removing for less, causing some window tokens
to never be removed.

The solution is to clear UID while calling
DisplayManager.getDisplays() API.

Bug: https://partnerissuetracker.corp.google.com/issues/383369807
Test: Run CTS tests that use UIAutomator/AccessibilityService and
      observe number of visible layers in SurfaceFlinger dumpsys
Change-Id: I5f207ddd09fab8c269db03d9a6dc09fb1d4cce92