Reland "Add weblayer browser_tests APK so we can run the tests on Android."

This is a reland of aaa2ec5c4603bfe5e423b8e3a49025c5fe75bd9f with fix from patchset 1 to 2 due to conflict with r702520.

Original change's description:
> Add weblayer browser_tests APK so we can run the tests on Android.
>
> The Shell C++ class isn't used on Android (unlike in content_shell) because we want to consume WebLayer there using the idiomatic Java API. At the same time, we want to write browser tests the same way for all platforms. So fake this by creating a Shell that reuses the existing BrowserController class that was created by Java.
>
> Change-Id: I946957f39eb79cff2e84c8abda56eed98b28d0a0
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815884
> Reviewed-by: Clark DuVall <cduvall@chromium.org>
> Commit-Queue: John Abd-El-Malek <jam@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#702563}

TBR=cduvall@chromium.org

Change-Id: I425b7b15d9f742a25a5b7c9dd306067ca29e551b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838953
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702657}

• BUILD.gn[diff]
• build/android/pylib/constants/__init__.py[diff]
• testing/android/native_test/BUILD.gn[diff]
• testing/android/native_test/java/src/org/chromium/native_test/NativeBrowserTestActivity.java[diff]
• weblayer/browser/browser_controller_impl.cc[diff]
• weblayer/public/browser_controller.h[diff]
• weblayer/shell/BUILD.gn[diff]
• weblayer/shell/android/browsertests_apk/AndroidManifest.xml[Added - diff]
• weblayer/shell/android/browsertests_apk/DEPS[Added - diff]
• weblayer/shell/android/browsertests_apk/src/org/chromium/weblayer_browsertests_apk/WebLayerBrowserTestsActivity.java[Added - diff]
• weblayer/shell/android/browsertests_apk/src/org/chromium/weblayer_browsertests_apk/WebLayerBrowserTestsApplication.java[Added - diff]
• weblayer/shell/android/browsertests_apk/weblayer_browser_tests_jni_onload.cc[Added - diff]
• weblayer/shell/browser/shell.cc[diff]
• weblayer/shell/browser/shell.h[diff]
• weblayer/shell/browser/shell_android.cc[Added - diff]
• weblayer/test/BUILD.gn[diff]