[ Merge of http://go/wvgerrit/65983 ]
Add support to drop closed metrics in order to save space for
long-running applications (and CdmEngine instances). The code now keeps
only a limited number of metrics collections after the session is closed.
As a session (and its metrics session) is closed, the oldest, closed metrics
session is dropped. This means those metrics will not be reported nor
accessible in the client.
Bug: http://b/118664842
Test: CDM Unit tests. Android Unit Test. Ran GPLay Manually.
Change-Id: I27d6e61a8fe4148ad1ef2a433c8e5f4cdd84cc72