Convert help texts (html files) to string resource.

Convert the localization and mechanism of displaying help texts from loading raw HTML files to using Android's first-class (localized) string resource.

As per guidance from upstream commit [714e8e3](https://github.com/schildbach/bitcoin-wallet/commit/714e8e3a1ca61f3f7a3f617450f63aebe39af2eb).

Note that using more compex HTML elements like `<ul>` and `<span>` is therefore not really supported in this model, but overall we gain in maintainability and in a clearer, more concise code and resource file layout.