[CDM][A11y] Fix talkback behavior for CDM device discovery dialog.

 * As new devices were found, the entire list was updated, which caused Talkback to get repeatedly interrupted while announcing device names. Only updating the newly added devices fixes this issue.
 * Adding accessibility live region to timeout messages allows status message to be automatically announced upon status update.

Bug: 396047983
Bug: 396314903
Test: Manually tested
Flag: EXEMPT bug fix
Change-Id: I32f6e6d75813db8593ed3f079e4a827746f9d9eb