Remove soundtrigger permission checks for cancelation operations

We should not require special permissions for unloading/stopping/
detaching. There are some scenarios where those operations would be
performed after the session owner had already lost its privileges,
such as when the designated assistant app gets replaced. The old app
should still be allowed to clean up.

Bug: 169628490
Test: atest GtsAssistIntentTestCases:com.google.android.assist.gts.KeyphraseModelManagerTest#testUpdateAndRemoveKeyphraseSoundModel
      Test still fails, but for different reasons.
Change-Id: Iea64970f7b2d7b387e6e892d555bbb90c73b1a18