Telephony: Add "singlepdp" RIL feature

ICS appears to assume the radio implementation is capable of multiple
simultaneous PDP contexts; maguro, for instance, brings up a second
interface (rmnet1) alongside the primary rmnet0 whenever another APN
is needed.
Other devices (including, from tests, crespo) don't accept this, and
react in unpredictable ways when a SETUP_DATA_CALL request is made before
tearing down any other active context (making it impossible, or at
least hard, to switch APNs).

Since the old setup/deactivateDefaultPDP methods are gone, this patch
adds a back-compatibility "singlepdp" option to the ro.telephony.ril.v3
property, which enforces disabling the default PDP whenever another one
is enabled.

Change-Id: Ib4b030e3f1a32445102ac72aeb68715ef8cebd88