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.
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f509803cad6652c79871bb0a7b509ba2d1054b67)
Merged-In: I24f0eafb2479ac7427ba28077e45ba26a9127cc8
Change-Id: I24f0eafb2479ac7427ba28077e45ba26a9127cc8