Don't install sdk variants, not platform variants

Background: if a lib has `sdk_version` set, it is mutated into two
variants: platform and sdk. The latter is for the inclusion into
unbundled APKs which can be installed on older platforms. The former is
what is installed to the platform.

So far, for unbundled app builds, (1) the platform variant was marked as
uninstallable, (2) while the sdk variant was left installable. (1) is
causing a problem (b/339262059) when an APEX containing a filesystem
module is built as unbundled. The filesystem skips installing the
platform variant. (2) is not causing a problem, but is unnecessary
because the sdk variant is not something to be installed (on the
platform). It is built into the APK via the jni_libs property. It's not
considered an installation.

This change fixes (1) and (2). For (1), the platform variant is always
marked installable even for unbundled build. For (2), the sdk variant is
always marked non-installable.. because, again, it's not designed to be
installed to the platform.

Bug: 339262059
Test: banchan com.android.virt aosp_arm64
UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true m apps_only dist
check libc++.so, libcrypto.so are in the microdroid image in the APEX.

Change-Id: I8999b724926cd9ab6d56796133e218fefcc1910b