MOTECH-589 User can select actual CommCare fields in Task Data source

This commit improves the DataProvider registered by the CommCare
module to include actual CommCare fields, instead of abstract
representation. The fields are determined based on the current
application schema. Each time an event about schema change
is received CommCare will rebuild the data sources and register
them again.

Change-Id: I97de185088727b46ff2bce3f24b109dd49663878
Merge "MOTECH-589 User can select actual CommCare fields in Task Data source"