[build] Build Mono.Android.Export.dll for the *minimum* API level

Commit 46de66f had the right idea, but *overlooked*
`Mono.Android.Export.dll`. (It corrected `OpenTK-1.0.dll`, which
suffers from the same problem described in 46de66f and needed fixing
as well, but it didn't actually address `Mono.Android.Export.dll`.)

Treat `Mono.Android.Export.dll` in the same way, updating
`make framework-assemblies` to build it for the minimum supported
framework version.

build-tools/scripts/BuildEverything.mk

@@ -110,6 +110,10 @@ framework-assemblies:
 		$(MSBUILD) $(MSBUILD_FLAGS) src/Xamarin.Android.NUnitLite/Xamarin.Android.NUnitLite.csproj /p:Configuration=$(conf) $(_MSBUILD_ARGS) \
 			/p:AndroidApiLevel=$(firstword $(API_LEVELS)) /p:AndroidPlatformId=$(word $(firstword $(API_LEVELS)), $(ALL_PLATFORM_IDS)) /p:AndroidFrameworkVersion=$(firstword $(FRAMEWORKS)); )
 	_latest_framework=$$($(MSBUILD) /p:DoNotLoadOSProperties=True /nologo /v:minimal /t:GetAndroidLatestFrameworkVersion build-tools/scripts/Info.targets | tr -d '[[:space:]]') ; \
+	$(foreach conf, $(CONFIGURATIONS), \
+		rm -f "bin/$(conf)/lib/xbuild-frameworks/MonoAndroid/$$_latest_framework"/Mono.Android.Export.* ; \
+		$(MSBUILD) $(MSBUILD_FLAGS) src/Mono.Android.Export/Mono.Android.Export.csproj /p:Configuration=$(conf) $(_MSBUILD_ARGS) \
+			/p:AndroidApiLevel=$(firstword $(API_LEVELS)) /p:AndroidPlatformId=$(word $(firstword $(API_LEVELS)), $(ALL_PLATFORM_IDS)) /p:AndroidFrameworkVersion=$(firstword $(FRAMEWORKS)); ) \
 	$(foreach conf, $(CONFIGURATIONS), \
 		rm -f "bin/$(conf)/lib/xbuild-frameworks/MonoAndroid/$$_latest_framework"/OpenTK-1.0.* ; \
 		$(MSBUILD) $(MSBUILD_FLAGS) src/OpenTK-1.0/OpenTK.csproj /p:Configuration=$(conf) $(_MSBUILD_ARGS) \