Fix vold wedge when unmounting Android/

In EmulatedVolume#doMount, if some operations fail, we call
EmulatedVolume#doUnmount.

During this unmount we try to unmount Android/ causing a FUSE_LOOKUP
on the FUSE mount. If the FUSE mount is not up, this can hang.

Now we introduce a new state to prevent unmounting Android/ if it
wasn't mounted.

Test: atest AdoptableHostTest
Bug: 151685786
Change-Id: I6246d3910c352034d2a4fb09ad9c1e7fd91cba5e