RIL: Add compatibility with qcom's DSDS

Devices with radio libraries built against a DSDS-enabled qualcomm
BSP (anything in the last 4 or 5 months, apparently) have a slight
incompatibility in the RIL protocol: it expects the first 4 bytes
of the datastream to be the slot's identification (even if the
device isn't dual-SIM). Absence of those 4 bytes will make the
connection hang.

Change-Id: I9f5821add02e7e5b5286c3b0818cb4a94edcc13c
Allow disabling radio with property persist.ril.enable.

This can be useful for phone with no active service.  It saves battery
and prevents unwanted communication with cell carriers.

Change-Id: I61ad66ee49384a53667066f426aedac1ab7d5d73