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
diff --git a/updater/Android.bp b/updater/Android.bp
index 872ef86..8a60ef7 100644
--- a/updater/Android.bp
+++ b/updater/Android.bp
@@ -81,6 +81,7 @@
         "blockimg.cpp",
         "commands.cpp",
         "install.cpp",
+        "mounts.cpp",
         "updater.cpp",
     ],