Basic support for the case that init_task.mnt_ns != zygote.mnt_ns(WSA),
just copy nsproxy and fs pointers for solve #276.
Note the copy in `apk_sign.c` is not required but suggested for
secure(ensure the checked mnt_ns is what ns android running, not created
by user, although many distributions does not have user ns.).
Tested with latest release on Win10 19045.3086(with WSAPatch).
Further review required for:
- [x] Security of this operation (without locking).
- [x] The impact of these modifications on other Android distributions.
c0066b68f5