[docs] Set $(AndroidApiLevel)=28 in Configuration.Override.props.in (#3149)

If you copy `Configuration.Override.props.in` to
`Configuration.Override.props` and don't change `$(AndroidApiLevel)`
from it's default value of 24, the `src/Mono.Android` build fails:

	obj/Debug/android-24/mcw/Java.Lang.Reflect.Parameter.cs(10,60): error CS0535: 'Parameter' does not implement interface member 'IAnnotatedElement.IsAnnotationPresent(Class)' 

The cause of this error is because the parameters to `api-merge.exe`
are "wrong"; see also: 0778165:

	mono --debug=casts ../../bin/BuildDebug/api-merge.exe -o "obj/Debug/android-24/mcw/api.xml" -s '../../bin/BuildDebug/api/api-*.xml.in' \
	  ../../bin/BuildDebug/api/api-10.xml.in … \
	  --last-description=../../bin/BuildDebug/api/api-28.xml.in

`api-merge.exe` uses `$(AndroidPlatformId)` to determine the Android
API-level to generate `api.xml` for, and thus uses the default value
of API-28 (from `Configuration.props`), while the subsequent
`generator.exe` invocation reads the API-28 `api.xml` and generates
code for `$(AndroidApiLevel)` (API-24), and this mismatch causes the
CS0535 error.

This is a classic Don't Do That™: the `$(AndroidApiLevel)`,
`$(AndroidFrameworkVersion)`, and `$(AndroidPlatformId)` MSBuild
properties *must* be synchronized with each other, but
`$(AndroidPlatformId)` *wasn't even mentioned* -- making it hard to
be kept in sync with the other values -- and (arguably) the comments
did not sufficiently warn anybody that these values must be
synchronized with each other.

Update `$(AndroidApiLevel)` and `$(AndroidFrameworkVersion)` so that
they correspond to the current latest stable version, so that if
someone inadvertently copies just one of the values things are *less*
likely to blow up.

Additionally, add `$(AndroidPlatformId)` and add a comment to attempt
to further clarify that these values must be kept updated together.

Author: jonpryor

Configuration.Override.props.in

@@ -1,10 +1,16 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
+    <!--
+      AndroidApiLevel, AndroidFrameworkVersion, and AndroidPlatformId MUST be
+      kept consistent with each other, lest Bad Things Happen™
+      -->
     <!-- The default Android API level to bind in src/Mono.Android -->
-    <AndroidApiLevel>24</AndroidApiLevel>
+    <AndroidApiLevel>28</AndroidApiLevel>
     <!-- The Xamarin.Android $(TargetFrameworkVersion) value that corresponds to $(AndroidApiLevel) -->
-    <AndroidFrameworkVersion>v7.0</AndroidFrameworkVersion>
+    <AndroidFrameworkVersion>v9.0</AndroidFrameworkVersion>
+    <!-- The default Android API "id" that corresponds to $(AndroidApiLevel) -->
+    <AndroidPlatformId>28</AndroidPlatformId>
 
     <!--
       Colon-separated list of ABIs to build the mono JIT for.