BlobStore: use a separate thread for RevocableFd in system_server.

Having the callbacks on RevocableFileDescriptor coming in on the main
thread of system_server can create problems:
- system_server's main thread is heavily contended
- it can cause deadlocks: callbacks come in from vold with vold's global
  lock held; this callback needs the system_server main thread to make
  progress. But if the main thread is busy with another call into vold
  (unrelated to RevocableFd), this will result in deadlock.

Bug: 300351508
Test: atest BlobStoreManagerTest
Change-Id: Ie4c3c65bdb9303f4aaab8f76b95d3f9f133b4c3e