[Xamarin.Android.Build.Tasks] Fix $(EmbedAssembliesIntoApk) (#876)

Commit 73dc058 removed the default for `$(EmbedAssembliesIntoApk)` if
it is blank. This was not the right thing to do :).

This commit adds the a default of 'True' is `$(EmbedAssembliesIntoApk)`
if completely emplty by the time it has gone through all the other
processes.

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -179,6 +179,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 	<AndroidUpdateResourceReferences Condition="'$(AndroidUpdateResourceReferences)' == ''">True</AndroidUpdateResourceReferences>
 	<EmbedAssembliesIntoApk Condition=" '$(EmbedAssembliesIntoApk)' == '' And '$(Optimize)' != 'True' And '$(_XASupportsFastDev)' == 'True' ">False</EmbedAssembliesIntoApk>
 	<EmbedAssembliesIntoApk Condition=" '$(_XASupportsFastDev)' == 'False' ">True</EmbedAssembliesIntoApk>
+	<EmbedAssembliesIntoApk Condition=" '$(EmbedAssembliesIntoApk)' == '' ">True</EmbedAssembliesIntoApk>
 	<AndroidPreferNativeLibrariesWithDebugSymbols Condition=" '$(AndroidPreferNativeLibrariesWithDebugSymbols)' == '' ">False</AndroidPreferNativeLibrariesWithDebugSymbols>
 	<AndroidSkipJavacVersionCheck Condition="'$(AndroidSkipJavacVersionCheck)' == ''">False</AndroidSkipJavacVersionCheck>
 	<AndroidBuildApplicationPackage Condition=" '$(AndroidBuildApplicationPackage)' == ''">False</AndroidBuildApplicationPackage>