[build] Build Xamarin.Android.NUnitLite.dll for API-28 (dotnet#2182)

We've been dealing with a head-scratching annoyance: `.apk` test would
execute, pass, and have their `TestResults.xml` files extractable when
the PR builds run the `.apk` tests, but *not* when `master` does.

This makes it difficult to know whether or not we've broken anything.

One "interesting" observation we've had is that the paths *differ* on
the "downloadable" vs. "undownloadable" builds.  For example,
[master build dotnet#1166][0] uses an internal storage path:

	RunTestApks
	  ...
	  Executing: /Users/builder/android-toolchain/sdk/platform-tools/adb   shell run-as Mono.Android_Tests cat "/data/user/0/Mono.Android_Tests/files/.__override__/TestResults.xml"
	    writing stdout to file: /Users/builder/jenkins/workspace/xamarin-android/xamarin-android/tests/../bin/TestRelease/TestResult-Mono.Android_Tests.xml
	…/xamarin-android/build-tools/scripts/TestApks.targets(188,5): error :   Command /Users/builder/android-toolchain/sdk/platform-tools/adb   shell run-as Mono.Android_Tests cat "/data/user/0/Mono.Android_Tests/files/.__override__/TestResults.xml" failed with exit code 1

While the "same" command on [PR build dotnet#4003][1] uses an external path:

	RunTestApks:
	  ...
	  Executing: /Users/builder/android-toolchain/sdk/platform-tools/adb -s emulator-5570  shell run-as Mono.Android_Tests cat "/storage/emulated/0/Android/data/Mono.Android_Tests/files/Documents/TestResults.xml"
	    writing stdout to file: /Users/builder/jenkins/workspace/xamarin-android-pr-builder/xamarin-android/tests/../bin/TestDebug/TestResult-Mono.Android_Tests.xml

*Why* do these paths differ?

The paths differ -- internal storage on master, external on PR builds
-- because `Xamarin.Android.NUnitLite.dll` only uses
[external storage directories][2] when it's built against a
`$(TargetFrameworkVersion)` value >= v4.4.

This is true on PR builds, where we only build one API level,
`$(AndroidFrameworkVersion)`, currently v9.0 (API-28).

This is *not* true on master builds where we build for *all* API
levels, and "really" build `Xamarin.Android.NUnitLite.dll` for the
[*first* supported API level][3], v2.3 (API-10)!

Update the `Xamarin.Android.NUnitLite.dll` build within
`make framework-assemblies` so that instead of building for the
*first* API level, we build for the *last* API level.

This should allow our unit tests to work sanely on "full" builds.

[0]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/1166/
[1]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android-pr-builder/4003/
[2]: https://github.com/xamarin/xamarin-android/blob/e153d42e51d36170487b3081c55310d978685b06/src/Xamarin.Android.NUnitLite/Gui/Instrumentations/TestSuiteInstrumentation.cs#L107-L110
[3]: https://github.com/xamarin/xamarin-android/blob/e153d42e51d36170487b3081c55310d978685b06/build-tools/scripts/BuildEverything.mk#L113-L118

build-tools/scripts/BuildEverything.mk

@@ -116,8 +116,8 @@ framework-assemblies:
 	$(foreach conf, $(CONFIGURATIONS), \
 		rm -f bin/$(conf)/lib/xamarin.android/xbuild-frameworks/MonoAndroid/v1.0/Xamarin.Android.NUnitLite.dll; \
 		$(_SLN_BUILD) $(MSBUILD_FLAGS) src/Xamarin.Android.NUnitLite/Xamarin.Android.NUnitLite.csproj /p:Configuration=$(conf) $(_MSBUILD_ARGS) \
-			/p:AndroidApiLevel=$(firstword $(API_LEVELS)) /p:AndroidPlatformId=$(word $(firstword $(API_LEVELS)), $(ALL_PLATFORM_IDS)) \
-			/p:AndroidFrameworkVersion=$(firstword $(FRAMEWORKS)) || exit 1; )
+			/p:AndroidApiLevel=$(lastword $(API_LEVELS)) /p:AndroidPlatformId=$(word $(lastword $(API_LEVELS)), $(ALL_PLATFORM_IDS)) \
+			/p:AndroidFrameworkVersion=$(lastword $(FRAMEWORKS)) || exit 1; )
 	_latest_stable_framework=$$($(MSBUILD) /p:DoNotLoadOSProperties=True /nologo /v:minimal /t:GetAndroidLatestStableFrameworkVersion build-tools/scripts/Info.targets | tr -d '[[:space:]]') ; \
 	$(foreach conf, $(CONFIGURATIONS), \
 		rm -f "bin/$(conf)/lib/xamarin.android/xbuild-frameworks/MonoAndroid/$$_latest_stable_framework"/Mono.Android.Export.* ; \