Remove PROP_FUSE_SNAPSHOT as it is no longer required

With the newly added flag for Settings developer options, which is now
used to change the state of FUSE, PROP_FUSE now acts as the snapshot
feature flag for the current boot.

Also, on a reboot PROP_FUSE_SNAPSHOT was always set to PROP_FUSE or
the default value (false) if PROP_FUSE is not set.

Since PROP_FUSE now replaces PROP_FUSE_SNAPSHOT, it always needs to be
set to some value after a reboot. PROP_FUSE is initialised to the default
value (false) only once, after that it takes value from the latest
changed flag.

Bug: 145391093
Test: Manual (adb shell ls /sdcard works)
Test: atest AdoptableHostTest
Change-Id: I3abb8690a50f542b2567c495ef9942a643be457d