[Android WebAPK] Make WebApkActivity translucent for new-style WebAPKs

This CL makes SameTaskWebApkActivity translucent. This is needed to
avoid visual artifacts during launch.

BUG=817263
R=dominickn
TBR=yfriedman

Change-Id: Ia3f132af7ed8a9db9b814e825068466c14b853e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611269
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659994}