[Android] Add shadows-core targets for multiple apis.

This added shadows-core-18.jar and shadows-core-21.jar targets.
This also fixes some build errors in the shadows-multidex target.

BUG=522043

Review-Url: https://codereview.chromium.org/2197403003
Cr-Commit-Position: refs/heads/master@{#409128}
5 files changed