Remove libpac as a JNI library in Android.mk

In fact, libpac is just a normal C++ library, not a JNI library.
Remove it from LOCAL_JNI_SHARED_LIBRARIES. libjni_pacprocessor
is the actual JNI library.

libpac is still being used. libjni_pacprocessor, specified in
frameworks/base/packages/services/PacProcessor/jni/Android.bp,
depends on libpac as shared library.

This change is needed to move libpac into the Runtime APEX.

Bug: 121269980
Test: m droid
Change-Id: I2a985acddbf2083d59fa28cfa76e6e4b80fa5b20