Run policheck as part of code analysis job

[pjcollins]

Context: https://github.com/xamarin/yaml-templates/pull/42

PoliCheck has been added to our code analysis job. Initial results from
running this tool reported 27 failures, which are all present in
external sources and have been added to an exclusion list.

All code anaylsis steps and reporting for the xamarin-android pipeline
previously lived in yet another template (xa-static-analysis/v2.yml),
but the content of that file is no longer well suited for a template.
Rather than creating another version of this template in the
yaml-templates repo, the core logic has been moved here.

[pjcollins]

Looks like 27 issues were found but the report stage was still green. We may need to create a suppressions list for this task. I'll see if I can get this to fail when issues are detected:
https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=3851603&view=logs&j=d3f8649d-db33-5c22-064d-51efdff82f35&t=37444108-2d0c-5c51-b8cb-04ce0fb93d42

[pjcollins]

Example of a build with failing PoliCheck results - https://github.com/xamarin/xamarin-android/runs/823241298

We should be able to merge this after the yaml-templates PR is merged.

[jonpryor]

Fixes: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1128206

Context: https://github.com/xamarin/yaml-templates/pull/42
Context: https://devdiv.visualstudio.com/DevDiv/_wiki/wikis/DevDiv.wiki/2704/Policheck
Context: https://microsoft.sharepoint.com/sites/globalreadiness/PublicDocuments/Forms/AllItems.aspx?id=%2Fsites%2Fglobalreadiness%2FPublicDocuments%2FPoliCheckSOMUserDocumentation%2FPoliCheckHelp%5Fv5%2E9%2E4%2Epdf&parent=%2Fsites%2Fglobalreadiness%2FPublicDocuments%2FPoliCheckSOMUserDocumentation

"Policheck is a text scanning tool designed to locate sensitive words
and phrases in Microsoft content".

Support for running Policheck has been added to our code analysis
job.  Initial results from running this tool reported 27 failures,
which are all present in external sources and have been added to an
exclusion list, `build-tools/automation/PoliCheckExclusions.xml`.

All code analysis steps and reporting for the xamarin-android pipeline
previously lived in the [`security/xa-static-analysis/v2.yml`][0]
template, but the content of that file is no longer well suited for a
template.  Rather than creating another version of this template in
the `yaml-templates` repo, the core logic has been moved here.

[0]: https://github.com/xamarin/yaml-templates/blob/d6db4a894be2271e3eb8f1faa1260c056cafa1c9/security/xa-static-analysis/v2.yml