Fix clickjacking again for accessibility services

Turns out that the flag for the window being obscured
does not imply that it is also partially obscured.

Also blocking system alerts and toast windows over the
accessibility service preference screen (and its associated
warning dialog) as well as the warning dialog shown when
a service is made available with the accessibiity shortcut.

Bug: 62104030
Test: Manually verified that I can't enable Select to Speak
when switch access is highlighting the button. I am able to
do that without this fix. Also started a service that
displayed a system overlay and confirmed that the overlay
disappears on a11y service preference screens and when a
new service is configured for the a11y shortcut.
Merged-In: Ie00bafa736c837881a258c9de10891b27c5baefd

Change-Id: Iabbded1a12dbc33d76e51c0bac710280a88080f3