plugins: minor cleanup of android_provision/apndb

Minor style cleanup of android-provision.c and
android-apndn.c.  Also fixed two minor bugs in
android-apndb.c:

1. Change attribute from "username" to "user" to
match the correct schema.

2. If an <apn /> element is found with a missing
"type" attribute, log an error, but don't fail
provisioning.
android-apndb: update context type handling

Return TYPE_INTERNET for apns with a 'type' attribute
prefixed with 'default'.  Return TYPE_MMS for apns with
'type=mms'.  Otherwise return TYPE_ANY.  Also updated
attribute handling, and added logic to strip any leading
"0"s from mmsproxy IPV4 addresses, so they're not
misconstrued as octal numbers by the ofono core gprs code.
src: allow MessageCenter/Proxy for TYPE_INTERNET

Allow a MessageCenter and MessageProxy to be set for an
OFONO_GPRS_CONTEXT_TYPE_INTERNET contexts in order to
support combined Internet/MMS contexts.