Check if sdcard daemon exited.

If the system is using sdcardfs, the sdcard daemon exits after mounting.
If it's using FUSE, the sdcard daemon runs until we go to unmount.

Bug: 37638548
Test: Run "adb shell ps | grep -w Z" with sdcardfs enabled.
      The sdcard daemon should not be listed.
      Run again with sdcardfs disabled. The daemon should be
      running, and vold should not be stuck waiting on it.
Change-Id: I930d22b35194ec99e7a6a4a022a04d36f4f39a34

EmulatedVolume.cpp

@@ -103,6 +103,8 @@ status_t EmulatedVolume::doMount() {
         LOG(VERBOSE) << "Waiting for FUSE to spin up...";
         usleep(50000); // 50ms
     }
+    /* sdcardfs will have exited already. FUSE will still be running */
+    TEMP_FAILURE_RETRY(waitpid(mFusePid, nullptr, WNOHANG));
 
     return OK;
 }

PublicVolume.cpp

@@ -190,6 +190,8 @@ status_t PublicVolume::doMount() {
         LOG(VERBOSE) << "Waiting for FUSE to spin up...";
         usleep(50000); // 50ms
     }
+    /* sdcardfs will have exited already. FUSE will still be running */
+    TEMP_FAILURE_RETRY(waitpid(mFusePid, nullptr, WNOHANG));
 
     return OK;
 }