[generator] Fix for fixing invalid annotation XML.

[jpobst]

Fixes #883

Due to invalid XML provided in Google's annotations.zip file, we run it through the more forgiving HtmlAgilityPack to attempt to fix it to valid XML.

However, given this snippet:

<item name="android.accounts.AccountManager android.accounts.AccountManagerFuture&lt;android.os.Bundle&gt; addAccount(java.lang.String, java.lang.String, java.lang.String[], android.os.Bundle, android.app.Activity, android.accounts.AccountManagerCallback<android.os.Bundle>, android.os.Handler)">
  <annotation name="androidx.annotation.RequiresPermission">
    <val name="value" val="&quot;android.permission.MANAGE_ACCOUNTS&quot;" />
    <val name="apis" val="&quot;..22&quot;" />
  </annotation>
</item>

The invalid unescaped < and > characters in the <item name='' attribute seem to tell HtmlAgilityPack not to expect any attribute strings to be properly escaped. Thus when it gets to the <val val='' attributes, it treats " as unescaped as well, thinking we want to value to be the literal string """. When it writes out the valid XML, it realizes it needs to escape the ampersand, and writes out ", which breaks our usage of these annotations.

The fix is to "unescape" every " to a " so that it will be escaped correctly when saved as valid XML.

[jonpryor]

This works, but I'd like "more". Testing locally, I see errors such as:

CannotChangeAttribute : Attribute 'Android.Runtime.RequiresPermissionAttribute' on 'Android.App.WallpaperManager.Clear(Android.App.WallpaperManagerFlags)' changed from '[RequiresPermissionAttribute("android.permission.SET_WALLPAPER")]' in the contract to '[RequiresPermissionAttribute("android.permission.SET_WALLPAPER")]' in the implementation.

which is kinda non-sensical: i have an attribute, and have the same attribute? (We dearly need do submit some UX improvements to that tool!)

So, what changed?

$ diff -u obj/Debug/monoandroid10/android-{27,28}/mcw/Android.App.WallpaperManager.cs
@@ -553,7 +553,7 @@
                // Metadata.xml XPath method reference: path="/api/package[@name='android.app']/class[@name='WallpaperManager']/method[@name='clear' and count(parameter)=1 and parameter[1][@type='int']]"
                [global::System.Runtime.Versioning.SupportedOSPlatformAttribute ("android24.0")]
                [Register ("clear", "(I)V", "GetClear_IHandler", ApiSince = 24)]
-               [global::Android.Runtime.RequiresPermission ("android.permission.SET_WALLPAPER")]
+               [global::Android.Runtime.RequiresPermission ("android.permission.SET_WALLPAPER")][global::Android.Runtime.RequiresPermission ("android.permission.SET_WALLPAPER")]
                public virtual unsafe void Clear ([global::Android.Runtime.GeneratedEnum] Android.App.WallpaperManagerFlags which)
                {
                        const string __id = "clear.(I)V";

We went from one attribute to two attributes, both with the same value.

Ideally, this wouldn't happen: we should only emit a given attribute once.

[jonpryor]

See also: dotnet/android#6424