4535 Update some plugins’ targetSdkVersions to 31 (cla: yes, p: path_provider, p: shared_preferences, platform-android, p: flutter_plugin_android_lifecycle, needs tests)

4543 [webview_flutter] Added contributing section to the README.md (cla: yes, p: webview_flutter, platform-android)

4544 [webview_flutter] Android implementation of loadFile and loadHtmlString methods (cla: yes, waiting for tree to go green, p: webview_flutter, platform-android)

4550 [webview_flutter] Pre-emptively ignore prefer_const_constructor warning. (cla: yes, waiting for tree to go green, p: webview_flutter, platform-android)