Add FallbackAssistant config flag, alert assistant to request perms

Created a config flag to enable or disable the FallbackAssistant.
If there is an active voice service that doesn't have notification listener
permissions, notify it to request permissions from the user.

Bug: 123095811
Test: manual
Change-Id: I069a1c5956d300960a3bb62178d54976e68b238b
2 files changed