optional default base for ValueMetric

For pulled atoms, if use_zero_default_base is set to true, and if use_diff, the first data
piece will assume 0 value as base.
This requires a successful previous pull to set mHasGlobalBase.
mHasGlobalBase is reset when condition changes to false or we skip more
than 1 bucket.

Bug: 120476027
Bug: 120129928
Test: unit test
Change-Id: Id01a7bf8796394777f02ba2c9bfdc860f528b98f