Condition VNDK check on TARGET_BUILD_APPS

Bug: 113560265
Test: (with the topic)
Test: mv prebuilts/vndk/v28 v28
Test: make TARGET_PRODUCT=aosp_arm TARGET_BUILD_APPS=Launcher3 TARGET_BUILD_VARIANT=userdebug
      --> build pass
Test: lunch aosp_arm-userdebug
      --> error: VNDK version 28 not found
Test: mv v28 prebuilts/vndk/v28
Test: lunch aosp_arm-userdebug
      --> no error
Change-Id: Ifb553a5ee4cec8b3f3b79aebdf824d64571fa817