Fix duplicate install targets from sdk variants in --skip-make mode.

This can manifest as duplicate "dependency missing" rules in builds
where SOONG_ALLOW_MISSING_DEPENDENCIES, TARGET_BUILD_APPS and
--skip-make are used together.

Test: art/tools/build_linux_bionic.sh dalvikvm
  with "export TARGET_BUILD_APPS=com.android.art" after the lunch line
  in build_linux_bionic.sh.
Bug: 157549171
Change-Id: I0e8dec0020f194cd265c73a55153555a51f6a426