Let Talkback announce to the user when changing dark/light theme

Screenshot:
https://screenshot.googleplex.com/B42zbNsdsA8AC23.png

Video:
https://drive.google.com/file/d/10rUKU76GpsJie8B9UFlFC_PckNX0LPlE/view?usp=sharing&resourcekey=0-N9fOs-f6WYVpfg5_3zQOww

Bug: 207738704
Test: manual
Change-Id: I361cd216f752886e8e2aa9b6ba6545517ba4f31a