SF: Only buildCachedSets if (mergeWithCachedSets)

Bug: 163076219
Test: make

When mergeWithCachedSets returns false, mLayers contains only CachedSets
with the current time_point. So no layers will be determined to be
"inactive" in buildCachedSets, so it will never create any Runs or an
mNewCachedSet.

Change-Id: Id305effec2e49530bd3c05804ee354ad2044ab56