Core: Fix trim in Android<4.1
Css: Don't check fill-opacity in Android 2.3, it's not supported
Event: Blacklist the onbeforeunload test in Android 2.3

Similarly to iOS, Android 2.3 has the onbeforeunload property on window
but doesn't support the event. This commit blacklists the test there as well.
Effects: Don't check fill-opacity in Android 2.3, it's not supported