[pytorch] consolidate android gradle build scripts (#39999)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39999
Cleaned up the android build scripts. Consolidated common functions into
common.sh. Also made a few minor fixes:
- We should trust build_android.sh doing right about reusing existing
`build_android_$abi` directory;
- We should clean up `pytorch_android/src/main/jniLibs/` to remove
broken symbolic links in case custom abi list changes since last build;
Test Plan: Imported from OSS
Differential Revision: D22036926
Pulled By: ljk53
fbshipit-source-id: e93915ee4f195111b6171cdabc667fa0135d5195
