Rename KEY_ALLOW_HOLDING_VIDEO_CALL_BOOL.

Per API feedback, rename to KEY_ALLOW_HOLD_VIDEO_CALL_BOOL and update
the API docs for clarity.

Test: Rebuild code.
Test: Run telephony unit tests.
Bug: 149299319
Change-Id: I36574865829951a3561f6445e3e83c78c48c4573