Declarative downloadable fonts

Implement support for downloadable font requests in xml. Given the
xml fonts feature in O, this adds support to not only declare
local font files as font resources, but also Downloadable fonts
from a fonts provider.

A provider returns a font family (of one or more files) given a
query, so the new attributes are added to the font-family tag.

Additionally, add support to pre-declare downloadable font resources
in the Android Manifest. These will then be fetched at app startup
time so they are available to use from the Typeface cache asap.

When retrieving downloadable fonts via resources, the cache is
checked to see if the font is already there and is used, otherwise
a request is sent to the provider and the default font is returned
as we need a result synchronously.

To do this, the developer declares an additional fonts xml resource
file with the list of fonts to preload and links it in the manifest
with a meta-data tag.

E.g.:

res/font/mydownloadedfont.xml

<font-family xmlns:android="http://schemas.android.com/apk/res/android"
        android:fontProviderAuthority="com.example.test.fontprovider"
        android:fontProviderQuery="myrequestedfont">
</font-family>

res/font/preloaded_fonts.xml

<?xml version="1.0" encoding="utf-8"?>
<font-family xmlns:android="http://schemas.android.com/apk/res/android">
    <font android:font="@font/mydownloadedfont" />
</font-family>

and in the AndroidManifest.xml

<meta-data android:name="preloaded_fonts"
    android:resource="@font/preloaded_fonts" />

Bug: 34660500, 34658116
Test: WIP, need to add more
Change-Id: I1d92555e115e241bf23b59e6f5c6cca6c7361de7