Use DEBUG_KEYSTORE variable as debug.keystore

By using the same key when signing, the user can easily upgrade to
newer dev versions without having to uninstall the app first.

.travis.yml

@@ -22,6 +22,19 @@ before_script:
   - sed -i -r
     -e 's/(<string name="app_name">[^<]+)/\1 (dev)/'
     android_app/app/src/main/res/values*/strings.xml
+  # Create keystore by running:
+  #   keytool -genkey -v -keystore debug.keystore -storepass android \
+  #     -alias androiddebugkey -keypass android -keyalg RSA -keysize 2048 \
+  #     -validity 10000 -dname "CN=Android Debug,O=Android,C=US"
+  #
+  # Then base64 encode it with:
+  #   base64 -w0 debug.keystore
+  #
+  # Then add this to Travis environment variables as DEBUG_KEYSTORE (make sure
+  # not to enable displaying value in build logs)"
+  - test -n "$DEBUG_KEYSTORE"
+    && echo "$DEBUG_KEYSTORE" | base64 -d > $HOME/.android/debug.keystore
+    || true
 
 script: cd android_app && ./gradlew testDebug assembleDebug