fix(magnifier): avoid crash when smashing magnifier shortcut button repeatedly

This avoids a race condition where we start receiving events and
trying to send them to the input filter before it's been
installed.

Fix: 290271948
Test: manual:
1. Turn on magnification shortcut button
2. Tap it repeatedly quickly for like 30 seconds
3. Should not crash.

Change-Id: I24f0eafb2479ac7427ba28077e45ba26a9127cc8