Move mounts.cpp from libotautil into libupdater.

All the active users of mounts.h now live in updater/.

Test: mmma bootable/recovery
Test: Run recovery_unit_test on taimen.
Test: Code search shows no reference to otautil/mounts.h in device dirs.
Change-Id: I6c35d2e403e92a0111102d00aa4773f4f524650e