[build] Update $(AndroidApiLevel)=26

Commit 759fd47 [broke the **macOS+msbuild** build][0]:

[0]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android-msbuild/387/

	error XA0001: Unsupported or invalid $(TargetFrameworkVersion) value of 'v7.1'. Please update your Project Options.

The cause of the problem is the `<ResolveSdks/>` task: if both
`$(AndroidApiLevel)` and `$(TargetFrameworkVersion)` are set, then
`$(AndroidApiLevel)` takes precedence, and we set
`$(TargetFrameworkVersion)` to the value that corresponds to
`$(AndroidApiLevel)`.

Commit 759fd47 updated `$(AndroidFrameworkVersion)` to v8.0, so
"normal" builds (`make all`) will only build `Mono.Android.dll` for
v8.0. However, 759fd47 forgot to update `$(AndroidApiLevel)` to 26 --
the API level corresponding to v8.0 -- which resulted in a mismatch
when building e.g. `Xamarin.Android.NUnitLite.csproj`:
`$(AndroidApiLevel)` was 25, which implies/requires
`$(TargetFrameworkVersion)`=v7.1, but we never built v7.1, which
resulted in the above XA0001 error.

Fix the problem by updating `$(AndroidApiLevel)` to default to
`$(AndroidLatestApiLevel)` -- which is 26 as of 759fd47 -- to help
avoid this mismatch again.

Author: jonpryor

Configuration.props

@@ -16,6 +16,9 @@
     <!-- *Latest* API level binding that we support; used when building src/Xamarin.Android.Build.Tasks -->
     <AndroidLatestApiLevel Condition="'$(AndroidLatestApiLevel)' == ''">26</AndroidLatestApiLevel>
     <AndroidLatestFrameworkVersion Condition="'$(AndroidLatestFrameworkVersion)' == ''">v8.0</AndroidLatestFrameworkVersion>
+    <!-- The API level and TargetFrameworkVersion for the default Mono.Android.dll build -->
+    <AndroidApiLevel Condition=" '$(AndroidApiLevel)' == '' ">$(AndroidLatestApiLevel)</AndroidApiLevel>
+    <AndroidFrameworkVersion Condition=" '$(AndroidFrameworkVersion)' == '' ">$(AndroidLatestFrameworkVersion)</AndroidFrameworkVersion>
   </PropertyGroup>
   <PropertyGroup>
     <AutoProvision Condition=" '$(AutoProvision)' == '' ">False</AutoProvision>
@@ -36,10 +39,8 @@
     <ManagedRuntimeArgs Condition=" '$(ManagedRuntimeArgs)' == '' And '$(ManagedRuntime)' == 'mono' ">--debug=casts</ManagedRuntimeArgs>
     <MonoSgenBridgeVersion Condition=" '$(MonoSgenBridgeVersion)' == '' ">5</MonoSgenBridgeVersion>
     <HOME Condition=" '$(HOME)' == '' ">$(HOMEDRIVE)$(HOMEPATH)</HOME>
-    <AndroidApiLevel Condition=" '$(AndroidApiLevel)' == '' ">25</AndroidApiLevel>
     <AndroidPlatformId Condition=" '$(AndroidPlatformId)' == '' ">$(AndroidApiLevel)</AndroidPlatformId>
     <AndroidPreviousFrameworkVersion Condition=" '$(AndroidPreviousFrameworkVersion)' == '' ">v1.0</AndroidPreviousFrameworkVersion>
-    <AndroidFrameworkVersion Condition=" '$(AndroidFrameworkVersion)' == '' ">$(AndroidLatestFrameworkVersion)</AndroidFrameworkVersion>
     <AndroidToolchainCacheDirectory Condition=" '$(AndroidToolchainCacheDirectory)' == '' ">$(HOME)\android-archives</AndroidToolchainCacheDirectory>
     <AndroidToolchainDirectory Condition=" '$(AndroidToolchainDirectory)' == '' ">$(HOME)\android-toolchain</AndroidToolchainDirectory>
     <AndroidMxeInstallPrefix Condition=" '$(AndroidMxeInstallPrefix)' == '' And '$(NeedMxe)' == 'true' ">$(AndroidToolchainDirectory)\mxe</AndroidMxeInstallPrefix>