android: load release signing from keystore.properties

Read upload-keystore credentials from a gitignored keystore.properties
(falling back to unsigned release when absent), bundle full native debug
symbols for Play crash symbolication, and ignore keystore secrets.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

android/.gitignore

@@ -11,3 +11,8 @@
 
 # Local build artifacts
 *.apk
+
+# Signing — never commit the keystore or its passwords
+keystore.properties
+*.jks
+*.keystore