Clear out service disabled reason on enable for V3 API

When a service gets re-enabled, clear out the 'disabled_reason' column.
Corresponding fix for the V3 API which was commited for the V2 api
in commit 16b4efdb0d488478c6f03218931bac4c42a278e3

Fixes bug 1212217

Change-Id: I3a5cc716f6c4929abaddf457b06f70f561789f38
Merge "Clear out service disabled reason on enable for V3 API"