add install step and docs for Android build (#17298)

add install step and docs for Android build (#17298)

Summary:
This commit did below enhancements:
1, add doc for build_android.sh;
2, add install step for build_android.sh, thus the headers and libraries can be collected together for further usage conveniently;
3, change the default INSTALL_PREFIX from $PYTORCH_ROOT/install to $PYTORCH_ROOT/build_android/install to make the project directory clean.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17298

Differential Revision: D14149709

Pulled By: soumith

fbshipit-source-id: a3a38cb41f26377e21aa89e49e57e8f21c9c1a39