Add null check to a11y interrupt.

Also adding same robustness to interrupt that we have for
sending a11y events.

Bug: 32507871

Test: Ran a11y CTS. Verified manually with sample app
that sends interrupt and accessibility service that
crashes when started. That case used to crash the
app, and doesn't anymore.
Change-Id: I5cf05dcbb54ea23ae876cb3258dd206c55dce775