[Bundles] Create Devtools DFM, hidden behind a flag.

This CL creates the Devtools DFM upstream, hidden behind a new flag
|dfmify_devtools| (default = false). The currently empty DFM is created
by following the instructions in
and also add an empty DevtoolsUi class. Details:
* DevtoolsUi is needed so dexsplitter.py and apkbuilder.py can have
  a valid DEX param for the Devtools DVM.
* DevtoolsUi uses @UsedByReflection so it won't be optimized away by
  ProGuard / R8.

Bug: 927131
Change-Id: Iefbe39f5825f8c1579f8b9c1a94d7196b1f44734
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613558
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662635}
10 files changed