Do not consider disabled prebuilt variants.

This relands https://r.android.com/1356826 without changes. It got
reverted as part of b/161315642 but is not related to that issue.

Test: m nothing
Test: `m` with prebuilt SDKs in the manifest
Bug: 151303681
Bug: 161315642
Change-Id: I7af892ae28a8208113e0801b1b8a8204edb61ec8