Make migrate option menu invisible when src volume is not mounted

When the adopted media storage is ejected, migrate operation from
adopted storage to internal storage always fails.

This CL makes the migrate option menu invisible when the source
volume of migration is not mounted and not accessible.

Bug: 117178027
Test: Settings robotests & manual
Change-Id: I8bfc9de552a8daf861315a5e46ee520ede14dd81