[xaprepare] Update Android 10 name (#4011)

[jonpryor]

Context: https://www.blog.google/products/android/evolving-android-brand/

The AndroidApiInfo.xml files produced by xaprepare during build
preparation includes a Name value:

    <AndroidApiInfo>
      <Id>29</Id>
      <Level>29</Level>
      <Name>Q</Name>
      <Version>v10.0</Version>
      <Stable>True</Stable>
    </AndroidApiInfo>

Both the version number and name are shown within various parts of
the IDEs, e.g.

    Android 9.0 (Pie)
    Android 8.1 (Oreo)

The problem is that starting with Android 10, Google is no longer
providing "nicknames" for Android releases.

What should Visual Studio do?

Currently, we still emit a Name of "Q" for API-29, thus VS shows:

    Android 10.0 (Q)

We could "clear out" the Name value, but unfortunately VS currently
doesn't special-case this in any way, and thus would show:

    Android 10.0 ()

Which looks "wrong".

Fixing this will require two changes:

1. We (xamarin-android) will begin clearing out the Name for Android
   releases which lack a nickname, and

2. The VS team will modify their Target Framework name generation
   logic such that the parenthesis and Name are only used when Name
   is a non-empty value. This will allow API-29 to have a displayed
   name of Android 10.0.

Begin the process: begin emitting an empty Name field for API-29,
allowing the IDE team to begin implementing their side.

[jonpryor]

Closing as "moot" as this is dealt with in the IDEs.