AW: add build.gradle to build boundary interfaces in AndroidX

Add build.gradle and AndroidManifest.xml files that are needed to build
these interfaces as an independent target in AndroidX repo.

These files are used by AndroidX build scripts only and should not be
used inside chromium project.

Bug: https://issuetracker.google.com/issues/140178227
Change-Id: I7686c7e49bfc023dd818210e583b7f535fe9b61c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1823119


Commit-Queue: Hazem Ashmawy <hazems@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700292}