Refactoring: pulled ServiceHandler methods to its parent class.

Now ServiceHandler only have 2 methods:

- handleMessage(): part of its interface.
- poll(): delegates work to parent, but sends a delayed message
  so it keeps polling.

Also changed hardcoded "N/A" to a resource.

BUG: 25794470
Change-Id: I486fff46c1532685bfd6f5903349d14e55059219