Improve battery saver setting a11y semantics

Adding the ability to override a couple a11y traits
of the Settings version of SeekBarPreference.
Setting semantics for the battery level to report
itself as a percentage. Also tweaking the content
description to use a different title. The title isn't
exactly what I'd like, but it is better than just
saying "At 15% - 15%".

Bug: 74357845
Fixes: 78479637
Test: Used a custom accessibility service to inspect
the information coming from the battery saver seek bar.

Change-Id: I9db55cf0bb3cfee86a46d22c1208262407332d5f