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)

Eventually we should write e2e test for a11y behaviour but currently e2e test for backlight indicator
is not yet viable because of driver issues, tracked as b/279899582

Test: Enable Talkback -> trigger backlight indicator -> notice announcement of "keyboard backlight indicator" and current level.
Test: Enable Talkback -> trigger backlight indicator -> change level -> notice announcement of new level
Fixes: 308407114
Change-Id: Ic1847b01c1d7b6ac928420052659688bf7ced7a8