Fixes Gson R8 crash (#1510)

* Keep missing state classes

* Remove SessionStack as it's not really required

Author: keianhzo

app/proguard-rules.pro

@@ -52,7 +52,7 @@
 # --------------------------------------------------------------------
 -keep class org.mozilla.vrbrowser.ui.widgets.WidgetPlacement {*;} # Keep class used in JNI.
 -keep class org.mozilla.vrbrowser.ui.widgets.Windows$** {*;} # Keep state clases used by gson.
--keep class org.mozilla.browser.engine.SessionStore** {*;} # Keep state clases used by gson.
+-keep class org.mozilla.vrbrowser.browser.engine.SessionState$** {*;} # Keep state clases used by gson.
 
 # --------------------------------------------------------------------
 # Keep classes from HTC SDK