ui: update proguard rules to keep crash logs readable

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

ui/proguard-rules.pro

@@ -1,3 +1,6 @@
 # Squelch all warnings, they're harmless but ProGuard
 # escalates them as errors.
 -dontwarn sun.misc.Unsafe
+
+# Retain some information to keep stacktraces usable
+-keepattributes SourceFile,LineNumberTable