[Xamarin.Android.Build.Tasks] Use Path.Combine() in <CalculateProjectDependencies/>

[brendanzagaeski]

Fixes: #4677

<CalculateProjectDependencies/> was always using / as the path
separator for the build-tools and platforms output items:

Task "CalculateProjectDependencies"
  Task Parameter:TargetFrameworkVersion=v10.0
  Task Parameter:ManifestFile=C:\Source\xamarin-android\bin\TestDebug\temp\InstallAndroidDependenciesTest\Project\Properties\AndroidManifest.xml
  Task Parameter:BuildToolsVersion=29.0.2
  Task Parameter:PlatformToolsVersion=29.0.5
  Task Parameter:ToolsVersion=26.1.1
  Task Parameter:NdkVersion=16.1
  Task Parameter:NdkRequired=False
  Output Item(s):
      AndroidDependency=
          platforms/android-29
          build-tools/29.0.2
                  Version=29.0.2
          platform-tools
                  Version=29.0.5
          tools
                  Version=26.1.1
Done executing task "CalculateProjectDependencies".

This meant the commercial <InstallAndroidDependencies/> task wouldn't
ever install the build-tools or platforms components on Windows
because none of the candidate IAndroidComponent.FileSystemPath values
that were available via the SDK installer matched the input
ITaskItem.ItemSpec values.

Fix this by changing <CalculateProjectDependencies/> to use
Path.Combine() instead of /.

Test changes:

InstallAndroidDependenciesTest did not yet enforce the expected
behavior for this bug because <ResolveSdks/> was automatically falling
back to the default Android SDK installation.

Update InstallAndroidDependenciesTest to enforce the expected
behavior.

To minimize download requirements for the test, add a mock
tools/source.properties file to the target android-sdk directory
during each test run. This reduces the total downloads from about 1 GB
to under 100 MB, and, because the tools component isn't needed for the
current default project configuration, b.Build (proj) still completes
successfully.

[dellis1972]

This must have changed behaviour in the sdk installer. I seem to remember the spec required the system to use / as a separator :/

[jonpryor]

Relevant spec: https://microsoft-my.sharepoint.com/:w:/p/mhutch/ETjdNgT_yfJMszzuRRW2hUYB133Goo831A5U2rAFwgI8bA?e=77b68c4ccf5841139a242a0d99cd0368

Forward slash / is explicitly mentioned in the spec:

For items in the main Android repository manifest, the ID shall be the path formatted in *nix format, e.g. “platforms/android-25” or “ndk-bundle”. For add-ons, the ID shall be the add-on’s “name-id” property. We will also have our own manifest format where the fileSystemPath will be exposed directly.

We do not want to use Path.Combine() here.

[dellis1972]

So in that case the issue must be in the task which calls the SDK Manager or the SDK Manager code itself I guess.

[dellis1972]

Interestingly on windows doing the following does not install anything

/t:InstallAndroidDependencies /p:AndroidSdkDirectory=c:\Sandbox\sdk /p:AcceptAndroidSDKLicenses=true

where the sdk folder is empty. I would expect it to install everything the project needs. This is what we are telling our customers, and it used to work. I have a feeling that something in the InstallAndroidDependencies or the sdk installer itself is not behaving correctly now. We should investigate that to fix the issue in the sdk-installer.

[dellis1972]

It works fine on a Mac.

[dellis1972]

This was fixed in https://github.com/xamarin/android-sdk-installer/pull/452 so we can close this PR.