Move MediaMetrics into its own library

Changes for the new HAL structure have brought about a build time
circular dependency between libmedia and libmediadrm. This set of
changes the media metrics code into its own library and arranges
for the relevant consumers of the media metrics code to link
to this new library (where they had previously picked it up by
including libmedia).

Bug: 36990807
Test: build, boot.
Change-Id: I81768eecca88d5e59e6e6e547f29308a94e30c7b