Prevent sdk variants from install

Platform variants should be available even when unbundled_build so that
unbundled build of com.android.virt apex can correctly have microdroid
image with system variants.

Bug: 268582372
Test: m (soong test)
Test: banchan com.android.virt aosp_arm64
      UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true m apps_only dist
      (microdroid should have the same contents)
Change-Id: I682e4f1f477f3024f7719dfaa67006ef335e0640