chore(ci): Skip backend system tests when updating Android modules (#5455)

chore(ci): Stop backend system tests from running on Android-only PRs (JAVA-519)

Prior to this commit, Android-only PRs were silently triggering 24 unrelated backend system test jobs (up to 10 min each). This update lets us skip those jobs when we don't need them.

Note: Every push to main still runs the full matrix.

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Co-authored-by: Cursor <cursoragent@cursor.com>