ril: track multiple call states

Track multiple active calls so that we don't reset the
CPATH when there is active call on hold. This fixes issue
with lost audio when switching between calls.

Change-Id: Ib4c6d5fed96c48f0f2a7b7fba70b286fcf472619

Conflicts:

	ril/telephony/java/com/android/internal/telephony/U2RIL.java