Guard against simultaneous calls.

1. If you get an incoming call we auto reject it if any calls exist which
are in any state of dialing out. (Tested with SIP/PSTN combination).
2. If you answer an incoming call, we automatically disconnect the
foreground call if it does not support being put on hold. (Tested with
SIP/Cdma combination).

- Also fix bug where we were allowing a child call to be set as the
foreground call.

Bug: 17323602
Change-Id: Idf448299a17c6e6d9a6ed2c148af8e4cc9c16594