AppFuseUtil: Call ForceUnmount before PrepareDir.

We'd previously call ForceUnmount after the call to PrepareDir,
which would sometimes fail because the userspace counterpart of a
FUSE FS that was previously mounted at that mountpoint has gone
away. This is usually reproducible after a runtime restart.

Bug: 128459728
Test: Loop (adb shell start; atest MediaStore_Images_MediaTest; adb shell stop;)
Change-Id: I38d3908487123614c338266f983afb04e3ed78d4