Surround platform-compat check with Binder.clearCallingIdentity()

Otherwise it fails permission check since it would use the caller's uid.

Bug: 148521528
Test: Post a background custom toast and observe it either posts the
      toast or shows a textual warning toast (before this change,
      neither would happen and we'd see a SE in logcat).

Change-Id: I338846bda08b8a002440dd3b1a43393f2ff25011