Added rule to keep static members of CallSpec class

The support class for BangleJS devices uses reflection to retrieve an array of the fields and used them to generate JSON which is sent to the devices. Without the new rule ProGuard breaks alerts for incoming calls in release builds.

See also: https://github.com/espruino/BangleApps/issues/976

app/proguard-rules.pro

@@ -18,6 +18,10 @@
 -keepclassmembers class nodomain.freeyourgadget.gadgetbridge.service.devices.pebble.webview.JSInterface {
     public *;
 }
+# Required for refection in BangleJSDeviceSupport
+-keepclassmembers class nodomain.freeyourgadget.gadgetbridge.model.CallSpec {
+    public static *;
+}
 -keepattributes JavascriptInterface
 
 # https://github.com/tony19/logback-android/issues/29