Installs any platforms required to build the Android apps in this job's workspace.

In an Android app project, test project or library project the platform version to build against must be specified in either the project.properties or default.properties file, by setting the target parameter.

This build step will search for these files in your workspace, determine which target platform(s) are required, and then ensure that they are installed. If the required platforms are already installed, no action is taken.
Therefore it should always be safe to run this build step before any build steps which compile an Android project.