Make keyboard backlight indicator more accessible

1. Adding dialog title to describe what this dialog is about instead of default generic "SystemUI"
2. Update content description whenever value changes (or is shown for the first time)

Test: Enable Talkback -> trigger backlight indicator -> notice announcement of "keyboard backlight indicator" and current level.
Test: Enable Talkback -> trigger backligh indicator -> change level -> notice announcement of new level
Test: KeyboardBacklightDialogTest
Fixes: 308407114
Change-Id: I57fcaf3332c3467aded014af960f21f1ec5b1105