Also, added missing build scripts for flatc on Windows and fixed
numerous issues in the flatbuffers/android/jni/include.mk makefile.

Tested:
Verified dependent project (motive) can generate flatbuffers headers
with these changes on Android.

Bug: 25780616

Change-Id: I73f0dec8f7a6a7eb4e71951a8f1cd3e0d343a9ff