deprecate RADIO constant, add getRadioVersion method

On many devices the radio version system property is only available
when the radio is on, which is frequently not the case when the static
initializers for the Build class are run (eg, if the system has just
booted).  This means RADIO is forever "unknown" on these devices.
Deprecate it and add a method to return the radio version instead.
Apps will still have to deal with getting a null back if the radio
version is currently unavailable.

Change-Id: I63528eae93e1b9d0f7cec5a382724d0391ba1104