[SB][Screen Chips] Make chips accessible.

Two accessibility changes:
1) Add content descriptions to each of the chip types
2) For the screen recording countdown chip, make the region assertive so
   that the countdown is announced.

Bug: 332662551
Flag: com.android.systemui.status_bar_screen_sharing_chips

Test: With flag disabled, enable TalkBack -> Trigger phone call ->
verify description is "Ongoing phone call"
Test: With flag enabled, enable TalkBack -> Trigger phone call ->
verify description is "Ongoing phone call"
Test: Enable TalkBack -> Share screen to app -> verify description is
"Sharing screen"
Test: Enable TalkBack -> Cast screen to other device -> verify
description is "Casting screen"
Test: Enable TalkBack -> Start screen record -> verify the countdown is
announced, and that "Recording" screen is announced once the recording
actually starts. (Note: With both the old screen record icon and the new
chip, one of the countdown numbers is sometimes missed because there's
too many other announcements happening at the same time. As long as most
of the countdown is announced and "Recording screen" is always
announced, this seems okay.)

Change-Id: I8f6c82ad819fc19427d91b2b3c3d80cc9f173ae8