GATT database refactor

Move logic and data responsible for GATT Discovery into separate class.
Create clean representation of GATT database, and attributes for
storage.
Minor service discovery improvements:
* don't send requests to explore services with start handle equal to end handle.
* don't try to discover characteristic if only one handle is left at end
  of service - it uses two handles, one for definition, one for value
* if secondary service is defined before service that includes it, it
  should now be properly discovered.

Test: Added DatabaseBuilderTest, manual tests with LE HID devices, Daydream
      Controller and Clips camera.
Bug: 67057055
Change-Id: I511f4ee7c56c8fa65f012de0b6d1291321840d11