Check a11y enabled before sending events from PiP

Bug: 64615719
Test: Ran YouTube, issued 'adb shell input keyevent 171',
then plugged in a USB mouse and moved the pointer over
the PiP window. No longer crashes the SystemUI.

Change-Id: Ib28921bdf1e1b2e06aa472340a5733a59702193d
1 file changed