Only allow system to apply relinquishTaskIdentity

Any malicious application could hijack tasks by
android:relinquishTaskIdentity. This vulnerability can perform UI
spoofing or spy on user’s activities.

This CL limit the usage which only allow system to apply
relinquishTaskIdentity

Bug: 185810717
Test: atest IntentTests
      atest ActivityStarterTests
      atest com.android.server.wm.TaskTests
Change-Id: Iddbb9c04237fdc8bdc0c2223ee597f43f37cb178