authfs: Remote file editor over binder
The remote editor allows to read/write from/to a remote file. This
change:
- Adds new binder API `writeFile`.
- Update fd_server to serve a read/writable fd. Read operation is done
through the existing read API when applicable.
- Adds `RemoteFileEditor` as a binder client for both read and write.
Bug: 171279640
Test: adb shell exec 9<>/data/local/tmp/output fd_server --rw-fds 9
Test: with changes in fusefs.rs, saw file changed correctly (md5sum)
Change-Id: I78ef198ee8a3a0f2d99717dc0c00fccde757f3de
4 files changed