Add shmem config buffer to libvrflinger

This allows VrCore to configure some tunable behaviors
of libvrflinger.

- Added dvrDisplayManagerDeleteNamedBuffer for testing config buffer
  creation.
- Added tests for named buffers, including one for the config buffer.
- Added IsValid to broadcast_ring to avoid tracking redundant state
  externally.

Bug: 38193993
Test: Run dvr_named_buffer-test
Change-Id: I52722dd314233b5bea1ca6377c14b5c856825746