vndk-def: Add sp-both-vndk-stable

This commit extracts sp-both-vndk-stable from sp-ndk-vndk-stable.
If a vndk-stable library is used by both SP-NDK and SP-HAL, then it
will belong to sp-both-vndk-stable.

Separating these libraries into another set makes it easier to:

1. Check whether a library is used by SP-HAL.
2. Estimate how many libraries will be loaded twice.

Test: Run vndk_definition_tool.py against sailfish image and only
sp-ndk-vndk-stable set is changed.

Change-Id: I2272086417c7dca515544520df5877bf6804a5e9