Global gesture to toggle Accessibility system-wide.

1. This change adds a global gesture for enabling accessibility.
   To enable this gesture the user has to allow it from the
   accessibility settings or use the setup wizard to enable
   accessibility. When the global gesture is enabled the user
   can long press on power to bring the global actions dialog
   and then hold with two fingers for a few seconds to enable
   accessibility. The appropriate feedback is also provided.

2. The global gesture is writing directly into the settings for
   the current user if performed when the keyguard is not on. If
   the keygaurd is on and the current user has no accessibility
   enabled, the gesture will temporary enable accessibility
   for the current user, i.e. no settings are changed, to allow
   the blind user to log into his account. As soon as a user
   switch happens the new user settings are inherited. If no
   user change happens after temporary enabling accessibility
   the temporary changes will be undone when the keyguard goes
   away and the device will works as expected by the current user.

bug:6171929

3. The initialization code for the owner was not executed due
   to a redundant check, thus putting the accessibility layer in
   an inconsistent state which breaks pretty much everything.

bug:7240414

Change-Id: Ie7d7aba80f5867b7f88d5893b848b53fb02a7537