Add session metric pruning.

[ 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

libwvdrmengine/cdm/core/src/cdm_engine.cpp

@@ -194,6 +194,7 @@ CdmResponseType CdmEngine::CloseSession(const CdmSessionId& session_id) {
     LOGE("CdmEngine::CloseSession: session not found = %s", session_id.c_str());
     return SESSION_NOT_FOUND_1;
   }
+  metrics_.ConsolidateSessions();
   return NO_ERROR;
 }