Engine: Request interception and content injection

[pocmo]

APIs

WebView

WebViewClient.shouldInterceptRequest
https://developer.android.com/reference/android/webkit/WebViewClient#shouldInterceptRequest(android.webkit.WebView,%20android.webkit.WebResourceRequest)

GeckoView

GeckoSession.NavigationDelegate.onLoadRequest

Implementations

Firefox for Fire TV intercepts firefox:home and injects a blank page:
https://github.com/mozilla-mobile/firefox-tv/blob/c660e1938eb0f112379e38c8af8a3f892ed9ba1f/app/src/amazonWebview/java/org/mozilla/focus/webview/FocusWebViewClient.java#L117-L127

Firefox for Fire TV injects a custom page for firefox:about URLs:
https://github.com/mozilla-mobile/firefox-tv/blob/c660e1938eb0f112379e38c8af8a3f892ed9ba1f/app/src/main/java/org/mozilla/focus/browser/LocalizedContent.kt#L24

Focus intercepts internal URLs (e.g. focus:about) and injects generated content:
https://github.com/mozilla-mobile/focus-android/blob/master/app/src/main/java/org/mozilla/focus/web/GeckoWebViewProvider.kt#L410-L414

┆Issue is synchronized with this Jira Task

[pocmo]

Moving this into the 0.21 sprint and assigning myself: This is needed for Firefox for Fire TV for various internal pages and I would like to get this in for the 0.21 release.