Add support for host build to pytorch_android native code (#27664)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27664
When ANDROID_ABI is not set, find libtorch headers and libraries from
the LIBTORCH_HOME build variable (which must be set by hand), place
output under a "host" directory, and use dynamic linking instead of
static.
This doesn't actually work without some local changes to fbjni, but I
want to get the changes landed to avoid unnecessary merge conflicts.
Test Plan: Imported from OSS
Differential Revision: D18210315
Pulled By: dreiss
fbshipit-source-id: 685a62de3c2a0a52bec7fd6fb95113058456bac8
