[Xamarin.Android.Build.Tasks] perf improvements for <ResolveAssemblies/>

[jonathanpeppers]

When building the SmartHotel360 app, I found that
<ResolveAssemblies/> was loading the NuGet ProjectAssetFile /
LockFile even though it was not used by the task at all. Putting a
Stopwatch around the code, I also found it to be somewhat expensive.

I reworked the code to use a Lazy<T> for accessing the LockFile.
This way it is not actually loaded until it is needed.

I also found several LogDebugMessage calls that seemed to be too
much... The <ResolveAssemblies/> was emitting hundreds of lines of
logging in the SmartHotel360 app.

I removed two messages such as:

LogDebugMessage ($"Adding {resolved_assembly} to topAssemblyReferences");
...
LogDebugMessage ("{0}={1}", assemblyName, apiLevel);

The [Input] and [Output] values for the task and the remaining log
message should be sufficient for understanding what we need from
customer logs:

LogMessage ("{0}Adding assembly reference for {1}, recursively...", new string (' ', indent), assemblyName);

The results for a build with no changes with the SmartHotel360 app:

Before:
320 ms  ResolveAssemblies                          1 calls
After:
238 ms  ResolveAssemblies                          1 calls

This saves ~82ms, and since this task runs on every Build and
Install, it could save ~164ms from the dev-loop.

[jonathanpeppers]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jonathanpeppers]

The build failures look to be on master, too: apk sizes, tls tests, designer, libZipSharp/DotNetPackageXASdkProject test.