[Media TTT] Add more info in talkback of receiver icon

Adds (Casting) before the app name in the content description to make
the talkback say more details about the UI event.

Bug: 265635955
Test: atest MediaTttUtilsTest
Test: Checked using adb commends. video with the fix in bug link.
Change-Id: I166cacd5959b01038bba1d96c2ef2be1a647e026