Add host build for pytorch_android (#27662)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27662
This adds a new gradle subproject at pytorch_android/host and tweaks
the top-level build.gradle to only run some Android bits on the other
projects.
Referencing Java sources from inside the host directory feels a bit
hacky, but getting host and Android Gradle builds to coexist in the same
directory hit several roadblocks. We can try a bigger refactor to
separate the Android-specific and non-Android-specific parts of the
code, but that seems overkill at this point for 4 Java files.
This doesn't actually run without some local changes to fbjni, but I
want to get the files landed to avoid unnecessary merge conflicts.
Test Plan: Imported from OSS
Differential Revision: D18210317
Pulled By: dreiss
fbshipit-source-id: dafb54dde06a5a9a48fc7b7065d9359c5c480795
