Rename vndk-stable to vndk-sp (SP=Same-process)

The concept of VNDK-stable set is gone because they no longer need to be
stable across several Android releases. Instead, they are just small set
of system libraries (other than Low-Level NDK) that can be used by
same-process HALs. They need to be stable only during an Android release
as other VNDK libraries. However, since they are eligible for double
loading, we still need to distinguish those libs from other VNDK
libraries. So we give them a name vndk-sp, which means VNDK designed for
same-process HALs.

Bug: 37139956
Test: booting successful with vndk-sp libs in /vendor/lib(64)?/vndk-sp
Change-Id: I892c4514deb3c6c8006e3659bed1ad3363420732