Update lock icon a11y click handler on a11y change

Previously, it would only update when the
lock icon view was attached; however, a11y
services can change while the view is attached.

Test: turn on tbon via adb while on the LS and
see that "double tap to activate" works
  adb shell settings put secure enabled_accessibility_services com.google.android.marvin.talkback/com.android.talkback.TalkBackService
adb shell settings put secure enabled_accessibility_services com.google.android.marvin.talkback/com.google.android.marvin.talkback.TalkBackService
  adb shell settings put secure accessibility_enabled 1
Fixes: 224530029
Fixes: 215294494

Change-Id: I490c2a5867ffcb2bac56968af038e298f30f0b8e