[Bugfix] Fix the problem that the home key can not work in some cases.

When closeSystemDialogs of ActivityTaskManagerService is called from the one way binder callback KeyguardExitResult, the getCallingPid of Binder will return 0, resulting in a NullPointerException in proc.isPerceptible().

Change-Id: I76b653ae08d47fe232677413382d60475a2cbc47