GD-Storage: Add GetPairedDevices() and IsPaired() APIs

* StorageModule::GetPairedDevices() returns a list of paired devices
  currently in config
* IsPaired() returns true if a ClassicDevice or LeDevice is paired
* Further generalize ConfigCache such that definition of persistent
  properties is supplied in the constructor

Bug: 162269750
Tag: #gd-refactor
Test: atest bluetooth_test_gd
Change-Id: I7d83c634ab48b3f72b3d658281c39e190789bbe4