authfs: support remote output directory

This change contains 3 major groups:
 - authfs/{aidl, fd_server}: new AIDL API and the service implementation
 - authfs/src: implement FUSE APIs for creating directory and file, by
               interact with the new service API as a client
 - authfs/tests, tests/: test coverage

A few notable changes that might help reviewing:

 - Now that both AuthFs and FdService struct is no longer immutable (in
   order to allow writable directory), their BTreeMap are now guarded by
   Arc<Mutex<_>>.

   * AuthFs::insert_new_inode and FdService::insert_new_fd are designed
     specifically to allow querying then mutating the map, which isn't
     trivial.

 - File and directory modes from the user program / VFS are currently
   ignored (just not to grow the change size).

 - Some shuffling of test paths to make it easy to clean up in tearDown.

Bug: 203251769
Test: AuthFsHostTest
Change-Id: I50f3f1ba8a3ebd969cf0f25a8feab2ec8cb1a2dc
diff --git a/authfs/tests/AndroidTest.xml b/authfs/tests/AndroidTest.xml
index 9deab5b..643e2b4 100644
--- a/authfs/tests/AndroidTest.xml
+++ b/authfs/tests/AndroidTest.xml
@@ -23,7 +23,7 @@
 
     <target_preparer class="com.android.tradefed.targetprep.RunCommandTargetPreparer">
         <option name="throw-if-cmd-fail" value="true" />
-        <!-- Prepare test directory. -->
+        <!-- Prepare test directories. -->
         <option name="run-command" value="mkdir -p /data/local/tmp/authfs/mnt" />
         <option name="teardown-command" value="rm -rf /data/local/tmp/authfs" />
     </target_preparer>
@@ -33,7 +33,7 @@
         <option name="abort-on-push-failure" value="true" />
 
         <!-- Test executable -->
-        <option name="push-file" key="open_then_run" value="/data/local/tmp/authfs/open_then_run" />
+        <option name="push-file" key="open_then_run" value="/data/local/tmp/open_then_run" />
 
         <!-- Test data files -->
         <option name="push-file" key="cert.der" value="/data/local/tmp/authfs/cert.der" />