Fix TaskbarAllApps and TaskbarDivider contianer talkback annoucement

This cl includes
	- setting content description on new taskbar containers for all apps and divider line, so their announcement is clear to user and not unlabelled.

Test: Manual
Bug: 378855202
Bug: 378845796
Flag: EXEMPT bugfix
Change-Id: I4650d1960a6208189b08b16ff541b2af3ed94cb3