[incfs] Make MountRegistry thread-safe + monitor events

1. Add total thread safety to MountRegistry
2. Add a check on each cached info access to make sure we're
   only using an up to date cache
3. Add a getter interface on the internal state - this will let
   IncrementalService to iterate over the existing mounts on a
   restart

+ minor code cleanups

Bug: 148432149
Test: atest libincfs-test + adb install megacity.apk
Change-Id: I548ba28964875ba3c9493b299b4e140f75da7f36
10 files changed
tree: 883fe894300357e5cf33a5b21c01d9562b8bae88
  1. incfs/
  2. libdataloader/
  3. .clang-format
  4. PREUPLOAD.cfg