Heed SkipInstall flag when adding install rules for vendor-snapshot.

The prebuilt-vs-source selection operates by setting SkipInstall on
either module. When the vendor-snapshot module visits all modules it
will visit both, and if it is a static library it will then add
duplicate install rules for both source and prebuilt unless it
looks at SkipInstall.

Test: `m nothing` in a tree with prebuilts/runtime present, that
  has been updated with a snapshot after https://r.android.com/1398472,
  which started to propagate vendor_available flags to prebuilts
  snapshots.
Bug: 151303681
Change-Id: Ie425f0115e4180b7f4a8e682696697d55d666845