Make A11yServiceConnection PendingIntent immutable.

Require that the PendingIntent be immutable so that a malicious app is
not able to hijack and mutate any of the details.

Bug: 154913130
Test: a11y CTS & unit tests
Change-Id: Ib794fa9e80b2a6c5562c66a0c17ea7c92c500e19
(cherry picked from commit a0b63007)