shouldn't allow dialing second call while first call still in progress

[Bill Filler]

rtm build 184 on krillin

Steps to reproduce:

- open dialer
- make a call to number 1
- while the first call is ringing but not yet answered, immediately either switch to keypad and dial another number (or press the contacts button and select a contact's number) and press the Call button

Expected results:
- both calls should be placed and the view should switch to the multiple calls view showing both as "calling.."
OR
- if this is not technically possible, then the UI for placing a second call (keypad and contacts buttons) should be disabled until the first call is active

Actual results:
- the second call is never placed and the UI never switches from the live call view of Call 1

[Tiago Salem Herrmann (tiagosh)]

I would avoid disabling the call button. I think users should still be able to enter USSD codes during a call, so the call button needs to be enabled all the time. Also, other protocols like sip might allow this use case (call another number while the current call isn't fully active).
There is another use case that we currently don't support but also needs the call button enabled, which is pressing the call button when no number was entered to fill the entry with the last called number.

[Oleg (arteniioleg)]

Related to https://bugs.launchpad.net/dialer-app/+bug/1588437