fix fs-misstrust on android
this is a followup to !652 (merged)
Arti would fail to get loaded on Android, because getgrnam_r
(and possibly other functions) is not provided by Bionic.
Then it would fail to run in the same way as on iOS due to a group-writable directory somewhere on the path to its datadir.
I've added some references on why not verifying group-writability of the path is safe in my opinion, both for iOS and Android.