Adds rate limit to checking byte size.

Since there is a separate guardrail for memory used by uid map, we
no longer add the memory from uid map with the memory per each
config's metrics. We also prevent the byte size check from happening
too frequently. In order to mock the MetricsManager, we refactor
some of the existing methods.

Test: Added unit-tests and verified they all pass on marlin.
Change-Id: I15cf105f7d95f4016fdb0443b0a33eebe862cafb