We pass the write end of a pipe to fd_server which it uses to signal
when it is ready to serve.
Bug: 201764092
Test: atest ComposTestCase
Change-Id: Id952e953e79f6cef9c8f0c847a303a3a854f2248
composd spawns odrefresh in its usual domain. odrefresh then spawns
fd_server in a different domain, and makes binder calls back to
composd to perform individual compilation steps.
fd_server is fairly generic, and part of the virt APEX, but this
instance is specific to composd (e.g. it has access to ART files), so
I named the domain composd_fd_server.
Bug: 186126194
Test: Run composd_cmd, artifacts generated
Change-Id: I5a431dd00b5b396a67021c618fc6edcfb25aa21b
