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

services/accessibility/java/com/android/server/accessibility/AccessibilityInputFilter.java

@@ -388,11 +388,19 @@ class AccessibilityInputFilter extends InputFilter implements EventStreamTransfo
     @Override
     public void onMotionEvent(MotionEvent transformedEvent, MotionEvent rawEvent,
             int policyFlags) {
+        if (!mInstalled) {
+            Slog.w(TAG, "onMotionEvent called before input filter installed!");
+            return;
+        }
         sendInputEvent(transformedEvent, policyFlags);
     }
 
     @Override
     public void onKeyEvent(KeyEvent event, int policyFlags) {
+        if (!mInstalled) {
+            Slog.w(TAG, "onKeyEvent called before input filter installed!");
+            return;
+        }
         sendInputEvent(event, policyFlags);
     }