[build] set file extension for common ToolExe values

[jonathanpeppers]

Running the -t:Install target with a local xamarin-android build on Windows, I got the error:

AndroidAdb
Errors
    Xamarin.Android.Common.targets(2641,3): error MSB6004:
    The specified task executable location "D:\android-toolchain\sdk\platform-tools\adb" is invalid.

This appears to only happen with a local of xamarin/xamarin-android because it specifies AdbToolExe=adb in Configuration.props.

It appears that most of our tasks just use the pattern:

protected override string GenerateFullPathToTool ()
{
    return Path.Combine (ToolPath, ToolExe);
}

Where if ToolExe was completely blank, it would fall back to the value:

protected override string ToolName => OS.IsWindows ? "adb.exe" : "adb";

Searching the web:

https://grep.app/search?current=2&q=ToolExe&words=true&filter[lang][0]=C%23&filter[lang][1]=XML

It seems most other tasks use ToolExe as-is, if it is passed in. Where most of the time the default value is blank.

So, I think the simple fix here, is to specify the file extension in Configuration.props for any of our $(FooToolExe) values.

I also removed $(AndroidToolExe) because it is unused and may not event exist anymore in the Android SDK.