Make the connection between implementation and llndk_library explicit

Instead of assuming a module with the .llndk suffix exists, add an
llndk_stubs property to every cc_library module that has a
corresponding llndk_library.  Also rename the llndk_library to have
an explicit .llndk suffix.

Bug: 170784825
Test: no changes to build.ninja (excluding comments) or Android-${TARGET_PRODUCT}.mk
Change-Id: I362bf9ce1e7df8cea65410ac35c02f347b235d25