Browsers on Android are not required to use Android's built in WebViews. And several alternative browser engines are offered by the platform[0].
Android has no specific restrictions in place. Although it is the path of least resistance to use the built in WebView, since you now don't need to deploy your own.
[0] Including: Blink (Amazon Silk, Chrome, Opera), WebKit (BlackBerry, Dolphin, et al), Gecko (Firefox, Minimo), NetFront (Blazer). At the moment WebKit derivatives seem to be most popular, but several browsers re-build it from the source rather than just using Android's WebKit components.
Android has no specific restrictions in place. Although it is the path of least resistance to use the built in WebView, since you now don't need to deploy your own.
[0] Including: Blink (Amazon Silk, Chrome, Opera), WebKit (BlackBerry, Dolphin, et al), Gecko (Firefox, Minimo), NetFront (Blazer). At the moment WebKit derivatives seem to be most popular, but several browsers re-build it from the source rather than just using Android's WebKit components.