Implement text zoom for Message View

* Add preference for default text size
* Move saveSettings logic into onPreferenceChange handler
* Per user tests, default setting is large (not "normal") for XL devices.
* Use setting in MessageView's WebView

TODO: Investigate zooming header (to/from/subject/etc) as well.

Bug: 2282390
Change-Id: If32ed3626244b046941a461f974b3dbdb535f592
1 file changed