Import broadcast_ring.h from GVR (reland)

This will be used as a transport by the pose plumbing.

Remove host_supported=true from the blueprint. Although this library is
portable C++11, some of the tests aren't, and I don't know how to test
the various SDK targets in order to properly #ifdef them.

Bug: 37102512
Test: m -j32 broadcast_ring_tests; broadcast_ring_tests

Change-Id: Ib91387f2feba9030d2e892dc5145e353b88bad45