geekdroid-firebase: allow to validate QueryPagingSource boundary key This is useful when ordering by @ServerTimestamp field. When the document hasn't been sent to server it can't be used as a boundary key has the server timestamp is null. This change allows client to make the check and avoid an IllegalArgumentException.

*.iml
.gradle
/local.properties
/.idea/workspace.xml
/.idea/libraries
.DS_Store
/build
/captures
.externalNativeBuild