This fixes a problem where a CdmEngine instance (and its sessions) could
be closed before its metrics could be collected. The change allows the
wv_content_decryption_module to extract metrics from instances about to
be closed. These are held until reported to the caller.
Test: Manually verified that collection is now occurring correctly. Also
added unit test: wv_cdm_metric_test.
This is a merge from wvgerrit/29069
Change-Id: If82bfd5cae3b72b9d14ab4741424a7ae7cc0a3a6
