[bt][host][fidl] Use fuchsia.bluetooth.PeerId and error syntax

* Changed all host.fidl methods to use fuchsia.bluetooth.PeerId rather
than string as the peer ID type.
* Changed all host.fidl methods that have a reply to use the FIDL error
syntax and the fuchsia.bluetooth.sys.Error type instead of
fuchsia.bluetooth.Status.
* Converted some of the multi-line awaits in integration tests to inline
where the former was unnecessary.
* Removed the GATT proxy setup in bt-gap's connection handling.

Bug: 35008
Test: bt-integration-tests; bt-host-unittests.
Change-Id: Icc1c38206b032d714628d86bbe5e5d8c89e28a97