activity: move loading URL to a dedicated method
webview: remove spurious final keyword
webview: use synchronized `cancelScrolling()` from timer

`cancelScrolling()` is a synchronized method just for this purpose. On
the other hand, `startScrolling()` does not need to be since it is only
called from the UI thread.
webview: don't use deprecated `getScale()`

Just record the appropriate value when triggering `onScaleChange()`
method.
style: use TimeUnit instead of constants to convert between time units