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/test/Android.mk

@@ -88,6 +88,10 @@ test_src_dir := ../core/test
 test_main := ../core/test/test_main.cpp
 include $(LOCAL_PATH)/integration-test.mk
 
+test_name := metrics_collections_unittest
+test_src_dir := ../metrics/test
+include $(LOCAL_PATH)/unit-test.mk
+
 test_name := policy_engine_constraints_unittest
 test_src_dir := ../core/test
 test_main := ../core/test/test_main.cpp