[ci] Install fewer Android SDK platforms on test agents. (#7874)

[Previously](#6874), we reduced the number of Android SDK Platforms that needed to be installed on CI test agents to `19,21,26,32,33`.  This can significantly reduce the amount of time needed to `xaprepare` an agent.

With most uses of Classic gone from `main`, retesting this reveals more platforms that can be removed.

Removing `19,21,26,32` saves up to 2 minutes of `xaprepare` time, particularly on Windows.

Note: API 27+ should be already installed on our images:
- [Mac preinstalled software](https://github.com/actions/runner-images/blob/main/images/macos/macos-12-Readme.md#android)
- [Windows preinstalled software](https://github.com/actions/runner-images/blob/main/images/win/Windows2022-Readme.md#android)

Author: jpobst

build-tools/automation/yaml-templates/setup-test-environment.yaml

@@ -12,7 +12,7 @@ parameters:
   installLegacyDotNet: true
   restoreNUnitConsole: true
   updateMono: true
-  androidSdkPlatforms: 19,21,26,32,33
+  androidSdkPlatforms: 33
 
 steps:
 - checkout: self