Disabled BSM mode switch when plugged in

Battery Saver mode can no longer be toggled
when the device is plugged in. Additionaly,
A content description was added to the switch
so that talkback says something when it is single
tapped.

Bug: 30083279
Change-Id: Id901598dbe13c443cae0b32769a2e3ef50428c08
(cherry picked from commit b9ab480a)