More robust image resizing (Issue 2207)

[Issue 2207](https://code.google.com/p/ankidroid/issues/detail?id=2207)

This should resolve the issues relating to image resizing introduced in the last release. The underlying issue appears to be the javascript executing before the webview is fully loaded, which ends up reporting incorrect window dimensions.

The behaviour is different on older platoforms so there are two workaround in there. An interesting thing to note: when webview switching is enabled ("safe display mode"), the problem can appear on any card. When it isn't enabled, the behaviour is only observed in the first card. I think we are doing the webview switching thing on the first card still even with the option turned off (something to look into one day).