[Xamarin.Android.Build.Tasks] DesignTimeBuild should use live build cache's if they exist.

[dellis1972]

Fixes #1501

If we have valid live cache files for the additional resources, the design time build should make use of them. However it should not create them since it would cause build problems for the live builds.

[jonathanpeppers]

I thought this might also fix https://devdiv.visualstudio.com/DevDiv/Xamarin%20Project%20Mgrs/_workitems/edit/594126

I figured out how to repro this issue on 15.6 (simplest way):

• git clean -dxf
• Add AndroidUseManagedDesignTimeResourceGenerator=True to the Xamarin.Forms.ControlGallery.Android project
• Open the XF sln and run the Xamarin.Forms.ControlGallery.Android project

So I add the changes in this PR manually to my VS 2017 install by editing C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets in an elevated Notepad++, and repeated.

Then it worked? I'll retry a few more times to make sure it is not a random issue.

[jonathanpeppers]

Trying a few more times, and I think that 594126 is only happening randomly.

Having trouble repro-ing with the fix removed, so hard to know if this fixes 594126 or not.

[dellis1972]

I think this is good to go.

[jonathanpeppers]

👍

[garuma]

Tested on my end by using an older monodroid build and only changing the lines this PR changed (i.e. not accounting for the most recent MSBuild changes done to xamarin-android in between) and it does fix the issue.