Allow accessibility services to send Intent.ACSD

For targetSdk < S. For S, they can use the new
GLOBAL_ACTION_DISMISS_NOTIFICATION_SHADE action to dismiss the
notification shade.

Bug: 159334261
Test: Send Intent.ACSD from an a11y service and verify shade is
      collapsed when targetSdk < S, and exception thrown when targetSdk
      S+.
Change-Id: I0e4be3faef8efa53a0b8a08263e842e0c4b1553e