Remove API-level guard on NeuralNetworks.h (#22429)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22429
Android NDK r20 removes the guard `(__ANDROID_API__ <= __ANDROID_API_O_MR1__)`, so we do it here also. There is insufficient reason to keep these decls undefined for earlier API levels. NDK r15 and earlier don't even define `__ANDROID_API_O_MR1__`, so the preprocessor defaults it to 0 and the guard evaluates as TRUE.
Reviewed By: smeenai, hlu1
Differential Revision: D16084105
fbshipit-source-id: f0857b3eb0573fe219f0d6c5e6583f89e2b5518f
