Abstractify old webkit api

[atsushieno]

As part of discussion at dotnet/android#1078 (comment) ,
we decided to make changes to those inconsistent methods that used to be
virtual and then became abstract, to become abstract even in old API level.

It is done at metadata fixup level.

(copying the commitmsg from dotnet/android#1304)

[atsushieno]

rebased onto the latest master.

[jonpryor]

@atsushieno: Do you have the HTML-based API diff handy?

[atsushieno]

Not an exact output, but it is on what my changes are based (reported as inter-api diff between 8.0 and 8.1 as I didn't waste time on building everything on master):

<h1>### API BREAK BETWEEN v8.0 and v8.1</h1>
<!-- start namespace Android.Webkit --> <div> 
<h2>Namespace Android.Webkit</h2>
<!-- start type WebHistoryItem --> <div>
<h3>Type Changed: Android.Webkit.WebHistoryItem</h3>
<p>Modified properties:</p>
<pre>
<div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> int Id { get; }
</div></pre>

</div> <!-- end type WebHistoryItem -->
<!-- start type WebSettings --> <div>
<h3>Type Changed: Android.Webkit.WebSettings</h3>
<p>Modified properties:</p>
<pre>
<div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> bool NavDump { get; set; }
</div><div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> bool PluginsEnabled { get; set; }
</div><div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> string PluginsPath { get; set; }
</div><div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> bool UseDoubleTree { get; set; }
</div><div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> bool UseWebViewBackgroundForOverscrollBackground { get; set; }
</div><div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> int UserAgent { get; set; }
</div></pre>
<p>Modified methods:</p>
<pre>
<div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> WebSettings.TextSize GetTextSize ()
</div></pre>

</div> <!-- end type WebSettings -->

</div> <!-- end namespace Android.Webkit -->
Makefile:122: recipe for target 'check-inter-api-level' failed

[jonpryor]

Context: dotnet/android#1304

[atsushieno]

Closing as per dotnet/android#1304 (comment)