BtifConfig: Prevent unprotected access to btif config cache

* btif_config_sections() used to allow unprotected access to internals
  of btif config cache that is normally protected by a mutex
* Changing that function to btif_config_get_paired_devices() as all
  users of that API only require paired devices
* Instead of returning a reference to the underlying list, the new API
  returns a copy of all paired devices, thus preventing any race
  conditon from happening when iterating through this vector

Bug: 67595284
Test: manual pairing test
Tag: #stability
Change-Id: If20db2c94cebb538f7c8d5c7c4b4c463dcfc1988
Merged-In: If20db2c94cebb538f7c8d5c7c4b4c463dcfc1988