Add "canDial" check.

For bug #3164802.

CallManager allow a new phone call only if ALL of the following are true:

- Phone is not powered off
- There's no incoming or waiting call
- There's available call slot in either foreground or background
- The foreground call is ACTIVE or IDLE or DISCONNECTED.

Change-Id: I0124d600fd8c63b8c608301f3889b3faec47f1db