Add signal-android to run_on_app_dump

Contents of README.google:
Name: Signal Android
URL: https://github.com/signalapp/Signal-Android
Version: NA
Revision: 91ca19f294362ccee2c2b43c247eba228e2b30a1
Date: Nov 5, 2020
License: GNU General Public License v3.0

Bug: 152155164
Change-Id: I7e25abcc3042b065de529ace087368295bc5825e
diff --git a/.gitignore b/.gitignore
index 1d9773a..a537cdd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -152,6 +152,8 @@
 third_party/opensource-apps/rover-android.tar.gz
 third_party/opensource-apps/santa-tracker
 third_party/opensource-apps/santa-tracker.tar.gz
+third_party/opensource-apps/signal-android
+third_party/opensource-apps/signal-android.tar.gz
 third_party/opensource-apps/sunflower
 third_party/opensource-apps/sunflower.tar.gz
 third_party/opensource-apps/wikipedia