Support call waiting/call swap on single phone.

Update logic to allow a second call (on a single phone) to be
answered and swapped into and out of the foreground.
Bug: 62086689
Test: Pair a phone, make two phone calls, swap them and verify proper
audio routing

Change-Id: I7ab5f368714d22c5649489540e6b71dd5b26288c
(cherry picked from commit 2cc339ea1b0cad6445bf71adf19d41407aa7f9e0)
2 files changed