Partial config update: predicates

This cl is part of the effort to allow partial config updates in statsd.
As with matchers, there are two main steps:
1. Determine update status of each predicate
2. Perform the update
   a. Copy over preserved conditions and create new ones for ones that
need to be replaced or are new.
   b. Update the tracker and predicate indices of preserved conditions
   c. Init all conditions to initialize new ones and determine the
current condition status

Added new unit tests, and use gmock to make container assertions more
readable.

Test: atest statsd_test
Change-Id: I74249431a9db2b409253d536044acc1fc3cf5c4c