plugins: add OFONO_APNDB_PATH logic to apndb code

A new environment variable, OFONO_APNDDB_PATH is queried
by the android-apndb code, if found, it overrides the
default path. This allows a modified apndb to be used for
testing.  Also added strlen checks for mmscenter and
mmsproxy logic.
src: fix combined gprs-context property logic

This change modifies the gprs combined context ( Internet/MMS )
logic such that the MessageCenter and MessageProxy properties
are only added to a combined context if non-empty.  This differs
from the MMS logic where these properties will be added even if
the values from the context are zero-length strings.
Merge pull request #67 from tonyespy/mms-prov

Mms prov