Update strikethrough to default to <s> tag

[cameronvoell]

Fixes part of wordpress-mobile/gutenberg-mobile#729

Description

This change updates the strikethrough button to default to <s> tag instead of <del> tag. This brings AztecEditor-Android inline with AztecEditor-iOS and HTML5 standards.

To Test

Case 1:

1. Press Strikethrough button and add some text, note strikethrough works correctly
2. Switch to HTML mode, note that Aztec shows <s></s> tag around strikethrough text where it would previously have a <del> tag

Case 2:

1. Switch to HTML mode, add some text with a <del></del> tag around it (or update text with <s></s> around it to be <del></del> instead).
2. Switch back to visual mode and note that text is still displayed as crossed out, as expected.

Make sure strings will be translated:

• If there are new strings that have to be translated, I have added them to the client's strings.xml as a part of the integration PR.

[mchowning]

Tested and everything works well. Thanks @cameronvoell ! 👍