Refactor CecMessageBuffer to live in its own file

This allows tests to use Mockito spies instead of anonymous classes to
override methods in HdmiControlService. Using a spy caused runtime errors
because CecMessageBuffer, as an inner class, would call methods on the
original, uninitialized instance of HdmiControlService instead of the spy.

Test: atest
Change-Id: If255e7b73b5f15c27ecd5dfb7a37993663f39a48