[build] Cleanup for specifying [min|target|max]SdkVersion in GN

crrev/c/1650265 enforced setting [min|target|max]SdkVersion in GN.
However, this is to strict for some dependencies, e.g. WebRTC.
Therefore, relax rules so that you can specify the SDK versions in both
GN and the AndroidManifest.xml. And the build system will assert that
these values match.

+ Update some SDK versions that had been changed inadvertently in
  crrev/c/1650265.

TBR=tedchoc@chromium.org,torne@chromium.org

Bug: 891996
Bug: 977417
Bug: 977328
Change-Id: I89b944c1f73a35ba8fcdafdf4bdf62508759811e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660361
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#671446}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 07f70727879b890bcdb8183036e895de375ca695
2 files changed