There are many subclasses of the Memory class and the overwhelming
majority of them don't need to be exposed externally. We move all of
them to internal headers except MemoryOfflineBuffer, which moves to a
separate header. This dramatically reduces the exposed API surface and
makes the code more modular.
Also, remove the Offline code from libbacktrace. It's not used any where.
Test: Unit tests pass, clean tree still builds
Change-Id: I55dacdf080daba0bfe65c1ad53a4b326bb482e83
6b95a0e999