[Android] remove android specific target (#5442)

* [Android] get rid of specific android targets

* change to AndroidApplication

* remove before/after targets

* MonoAndroid

.nuspec/Xamarin.Forms.Android.targets

@@ -1,13 +0,0 @@
-<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-  <ItemGroup>
-    <ProguardConfiguration Include="$(MSBuildThisFileDirectory)proguard.cfg" />
-  </ItemGroup>
-  <Target Name="ValidateTargetFrameworkVersionForForms" BeforeTargets="CoreCompile" Condition="'$(XFDisableTargetFrameworkValidation)' != 'True'">
-    <PropertyGroup>
-      <MinTargetFrameworkVersionForForms>8.1</MinTargetFrameworkVersionForForms>
-      <TargetFrameworkVersionWithoutV>$(TargetFrameworkVersion.TrimStart('vV'))</TargetFrameworkVersionWithoutV>
-    </PropertyGroup>
-    <Error Code="XF005"  Condition="$(TargetFrameworkVersionWithoutV) &lt; $(MinTargetFrameworkVersionForForms)"
-           Text="The %24(TargetFrameworkVersion) for $(ProjectName) ($(TargetFrameworkVersion)) is less than the minimum required %24(TargetFrameworkVersion) for Xamarin.Forms ($(MinTargetFrameworkVersionForForms)). You need to increase the %24(TargetFrameworkVersion) for $(ProjectName)."   />
-  </Target>
-</Project>

.nuspec/Xamarin.Forms.nuspec

@@ -154,7 +154,6 @@
     <file src="..\Xamarin.Forms.Xaml.Design\bin\$Configuration$\Xamarin.Forms.Xaml.Design.dll" target="lib\tizen40\Design" />
     
     <!--Android 10-->
-    <file src="Xamarin.Forms.Android.targets" target="build\MonoAndroid10\Xamarin.Forms.targets" />
     <file src="proguard.cfg" target="build\MonoAndroid10\proguard.cfg" />
     
     <!--Android 81-->

.nuspec/Xamarin.Forms.targets

@@ -143,4 +143,36 @@
  			<Compile Include="@(_CSSOutputs)" />
  		</ItemGroup>
 	</Target>
+  
+ <!-- 
+    Platform Specific Targets.
+    We can't use a framework specific targets file because it breaks shared projects i.e.  /build/MonoAndroid10/Xamarin.Forms.targets
+  -->
+
+  <!-- MonoAndroid targets. -->
+  <PropertyGroup Condition="'$(TargetFrameworkIdentifier)' == 'MonoAndroid'">
+    <CoreCompileDependsOn>
+      IncludeProguardForAndroid;
+      $(CoreCompileDependsOn);
+    </CoreCompileDependsOn>
+    <PrepareForBuildDependsOn>
+      ValidateTargetFrameworkVersionForForms;
+      $(PrepareForBuildDependsOn);
+    </PrepareForBuildDependsOn>
+  </PropertyGroup>
+  
+  <Target Name="IncludeProguardForAndroid">
+    <ItemGroup>
+      <ProguardConfiguration Include="$(MSBuildThisFileDirectory)MonoAndroid10\proguard.cfg" />
+    </ItemGroup>
+  </Target>
+
+  <Target Name="ValidateTargetFrameworkVersionForForms" Condition="'$(XFDisableTargetFrameworkValidation)' != 'True'">
+    <PropertyGroup>
+      <MinTargetFrameworkVersionForForms>8.1</MinTargetFrameworkVersionForForms>
+      <TargetFrameworkVersionWithoutV>$(TargetFrameworkVersion.TrimStart('vV'))</TargetFrameworkVersionWithoutV>
+    </PropertyGroup>
+    <Error Code="XF005"  Condition="$(TargetFrameworkVersionWithoutV) &lt; $(MinTargetFrameworkVersionForForms)"
+           Text="The %24(TargetFrameworkVersion) for $(ProjectName) ($(TargetFrameworkVersion)) is less than the minimum required %24(TargetFrameworkVersion) for Xamarin.Forms ($(MinTargetFrameworkVersionForForms)). You need to increase the %24(TargetFrameworkVersion) for $(ProjectName)."   />
+  </Target>
 </Project>

Xamarin.Forms.sln

@@ -26,7 +26,6 @@ EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = ".nuspec", ".nuspec", "{7E12C50D-A570-4DF1-94E1-8599843FA87C}"
 	ProjectSection(SolutionItems) = preProject
 		.nuspec\proguard.cfg = .nuspec\proguard.cfg
-		.nuspec\Xamarin.Forms.Android.targets = .nuspec\Xamarin.Forms.Android.targets
 		.nuspec\Xamarin.Forms.AppLinks.nuspec = .nuspec\Xamarin.Forms.AppLinks.nuspec
 		.nuspec\Xamarin.Forms.Debug.targets = .nuspec\Xamarin.Forms.Debug.targets
 		.nuspec\Xamarin.Forms.DefaultItems.props = .nuspec\Xamarin.Forms.DefaultItems.props