Don't install stubs

Stubs libs are build-time only artifact. Can't be installed.

Test: OUT_DIR=out build/soong/scripts/build-ndk-prebuilts.sh
with https://android-review.googlesource.com/c/platform/frameworks/av/+/846770/7
and two more CLs in the same chain
Examine out/soong/build.ninja
Build rule for out/soong/target/product/generic_arm64/system/lib64/arm64/libmediandk.so
exist only once.

Change-Id: I58ff09ef3fbbdbd2d968aa21b87a493192d4216d