[create-pkg] Clean up previous installations

Adds a preinstall script which will completely delete our primary
installation directory of /Library/Frameworks/Xamarin.Android.framework
before the pkg is installed. These changes ensure that we don't leave
old versions behind when a new version is installed.

Author: pjcollins

build-tools/create-pkg/create-pkg.targets

@@ -15,6 +15,7 @@
     <PkgDistributionDestination>$(OutputPath)\pkg\distribution.xml</PkgDistributionDestination>
     <PkgLicenseSrcEn Condition="'$(PkgLicenseSrcEn)' == ''">$(XamarinAndroidSourcePath)\LICENSE</PkgLicenseSrcEn>
     <PkgLicenseDestinationEn>$(PkgResourcesPath)\en.lproj</PkgLicenseDestinationEn>
+    <PkgScriptsDir>$(MSBuildThisFileDirectory)scripts</PkgScriptsDir>
     <UpdateInfoGuid>d1ec039f-f3db-468b-a508-896d7c382999</UpdateInfoGuid>
   </PropertyGroup>
   <Target Name="_CopyFilesToPayloadDir"
@@ -120,6 +121,7 @@
       <PkgBuildArgs Include="--identifier com.xamarin.android.pkg" />
       <PkgBuildArgs Include="--version $(XAVersion)"/>
       <PkgBuildArgs Include="--install-location &quot;$(PkgInstallDir)&quot; "/>
+      <PkgBuildArgs Include="--scripts &quot;$(PkgScriptsDir)&quot; "/>
       <PkgBuildArgs Include="&quot;$(PkgOutputPath)/xamarin.android.pkg&quot; "/>
     </ItemGroup>
     <Exec Command="pkgbuild @(PkgBuildArgs, ' ')" />

build-tools/create-pkg/scripts/preinstall

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+DESTINATION=/Library/Frameworks/Xamarin.Android.framework
+
+# Ensure installations of previous versions are cleaned up 
+# by deleting all content in our installation directory root.
+if [ -d "$DESTINATION" ]; then
+	rm -rf "$DESTINATION"
+fi