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

Conflicts:
	src/java/com/android/internal/telephony/SamsungCDMAv6RIL.java
	src/java/com/android/internal/telephony/SamsungExynos3RIL.java