Android: Add vulnerability_reports to gitignore

Vulnerability reports are not meant for checking in but just for local
viewing.

Change-Id: If0f7d6eb905564542a0d365053e8adecf12dcab5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2323502
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Adrian Taylor <adetaylor@chromium.org>
Auto-Submit: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Adrian Taylor <adetaylor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792280}

third_party/android_deps/.gitignore

@@ -8,3 +8,6 @@ buildSrc/build/
 # JAR and AAR files will be fetched via CIPD.
 libs/*/*.jar
 libs/*/*.aar
+
+# vulnerability reports are for local viewing only.
+vulnerability_reports/*