Make ndk_prebuilt_*_stl's libDir relative to module

Currently, we have ~4 of these modules which are all defined in
prebuilts/ndk. However, using ctx.ModuleDir() instead has the following
advantages
- makes bp2build simpler since we do not need to relativize this path
- prevents soong modules from reaching into another directory (as the
  test setups were doing).

Test: m nothing
Change-Id: I780e2564cb37ebf4b800f0cd184789f3fc6f2fc8