Clean up base request object and subclasses

This change adds a type metadata field to the base request object that
request implementations will use to indicate what type of request they
implemenet. This will allow future introspection on the objects to tell
what they are without instanceof.

This change also adds a helper getResponseCode() function to expose the
return code of a given request.

These together are used in a new toString() implementation to make
printing request objects more info rich.

Request subclasses now report a type. A few unnecessary log lines have
been removed.

Flag: EXEMPT, mechanical refactor, no logic change
Bug: 365626536
Test: atest com.android.bluetooth.pbapclient
Change-Id: Ia3a4905b5fb9607b0973da12541678b9f26a2560