Maintain the fuse buffer pool keyed by inode instead of threadId.

In order to save memory, FuseAppLoop maintains a buffer pool to use
when dispatching the read/write requests. Currently, it uses
the threadId of the ProxyFileDescriptorCallback as the key for this
buffer pool and this can result in an issue when a caller creates
multiple ProxyFileDescriptors with ProxyFileDescriptorCallbacks
running on the same thread. When this happens, it is possible that
a buffer is reused before a read/write request which was using it
earlier has been handled and would result in data from read/write
requests on different fds getting mixed up.

Bug: 158568683
Test: atest --test-mapping apex/blobstore
Change-Id: Iaffd54f4430ecb3778f01c80f5a1684ac448f8dc