Polish of the accessibility settings.

1. Some string changes according to updated UX specs.

2. Explore by touch now shown only if accessibility is on.

3. The one time dialog for installing TalkBack if no services
   are installed was not showing.

bug:5293099
bug:5286578

Change-Id: I633999f798b16e2910d95d541a7c33d1a4f296b9