[wptrunner] Add a new argument for the wpt cli to pass custom Android activity names

[SamuelWeiss]

What are these changes
This supports plumbing custom activity names for running WPT on Android applications.

Why make these changes
Currently, it is only possible to run WPT for Android webviews when the activity name is exactly org.chromium.webview_shell.WebPlatformTestsActivity. This makes it complicated to run WPT on webviews in applications other than the reference activity provided by Chromium.

How to use this functionality
./wpt run --package-name com.my.application --activity-name com.my.application.MyActivity android_webview

Potential modifications
As you can see in the above example, the package name is repeated (com.my.application). It might be simpler to compose the activity_name by concatenating the package_name and just the final activity name (MyActivity). I avoided this design because it would change the existing behaviour for users who already override the package name.