Revert part of "Stop versioning NDK stubs pre-M."

This reverts commit e67144e3.

Keep the getApiLevelsMap function and keep using it in
shouldUseVersionScript. It seems useful. (Also, keeping it probably avoids
an AOSP->internal merge conflict...)

Bug: https://github.com/android-ndk/ndk/issues/750
Test: m out/soong/ndk.timestamp, examine dynsym in stubs
Change-Id: Icbb26fc46a9a1dc99f22af195468f08713c4f216