Improve MessageWebView display.

Now that MessageWebView has 'setUseWideViewPort(true)',
the wide view port is excessively wide.  It turns out
Android is using a fixed width of 980 px, so that even
plain text messages (which are already wrapped to fit
the screen) have a large empty area beside them when
scrolled to the left.

Injecting a meta tag in the html header fixes the
problem.
Change when <html> tags are applied to messages.

Previously, <html>, <head>, & <body> tags were
attached to messages before they were stored locally.
But now that the <head> element also needs to include
a <meta> element (for proper MessageWebView display),
it seems unecesary to store all these tags with each
message.

Now the tags are no longer stored with the messages.  Instead,
MessageWebView applies the tags before displaying the message.

This also eliminates the need to upgrade an older
message database where all the old messages would have
otherwise needed to be wrapped with the new tags.
Remove unused code in HtmlConverter.

HtmlConverter.getHtmlHeader() and getHtmlFooter() are
no longer used.  Remove them and other related code.
Refactor code for MessageWebView.setText()

The MIME type for the supplied text was always text/html,
so there is no need to pass that as a parameter.
Furthermore, we are relying on it being text/html because
we are wrapping it with HTML code.

Likewise, change/simplify/rename AccessibleWebView.loadDataWithBaseURL().
Fix unit tests.