Context: https://www.getcodetrack.com/
I used a "freeware" .NET profiler to see what it would turn up.
Using the tool, I profiled the following .NET process:
    .\bin\Debug\bin\xabuild.exe .\tests\Xamarin.Forms-Performance-Integration\Droid\Xamarin.Forms.Performance.Integration.Droid.csproj /v:quiet
This seemed to have a lot of useful information, and I quickly noticed:
    Monodroid.AndroidResource.UpdateXmlResource -> 3683 calls -> 2.13s
    ..
        Monodroid.AndroidResource.ResourceNeedsToBeLowerCased -> 2384 calls - 319.53ms
This is a known codepath, the `ConvertResourcesCases` MSBuild task,
that we know takes a bit of time.
Looking through the call stack, I saw two points we could make easy
fixes for:
- A `Regex` in `AndroidResource` was not using `RegexOptions.Compiled`
- There was a string append `dirPrefix + "*"` in a LINQ expression
  that could be done up front.
- There was a bit of LINQ usage such as:
    if (Directory.EnumerateDirectories (resourceBasePath, dirPrefix + "*").Any (dir => Directory.EnumerateFiles (dir, fileNamePattern).Any ()))
So I added `RegexOptions.Compiled` and converted the LINQ expressions
to `foreach` loops. I could see a noticeable change in these methods
which moved further down the list of methods, when sorted by time
duration.
But since the time differences *in the profiler* likely aren't going
to reflect the real world, I did a before/after comparision with the
Xamarin.Forms.ControlGallery project.
Project here:
https://github.com/jonathanpeppers/Xamarin.Forms/tree/msbuild-timing/Xamarin.Forms.ControlGallery.Android
Script here:
https://github.com/jonathanpeppers/msbuild-timing/blob/master/xamarin.forms.ps1
Before:
    7224 ms  ConvertResourcesCases                      6 calls
After:
    7034 ms  ConvertResourcesCases                      6 calls
So about a 200ms improvement on Windows, for adjusting a bit of code.
I suspect the improvement won't be as good on MacOS, since I don't
believe `RegexOptions.Compiled` is implemented in Mono. (It is safely
ignored)
Other changes:
- Code such as `(value ?? String.Empty).Trim ();`
- Simplified to `value?.Trim ();`
- Removed unused `filePath` variable
Future changes:
It may be worth auditing all regexes and adding
`RegexOptions.Compiled` if they appear to be called a lot?