Do not send KEYCODE_VOICE_ASSIST to apps.

Prevent KEYCODE_VOICE_ASSIST from being sent
to the apps. Move the key handling logic from
interceptKeyBeforeDispatching to
interceptKeyBeforeQueueing for KEYCODE_ASSIST
and KEYCODE_VOICE_ASSIST.

Allow the launch of assistant with KEYCODE_ASSIST

Remove dead link from the docs.

Test: bit CtsViewTestCases:.KeyEventInterceptTest
Bug: 64776073
Change-Id: I6ab596ffb0448923d71da3fc93ccdd26de8d517f