HFP: Print human readable event and state name by default

* Debug log in HFP's BTA code is gated by BTA_AG_SCO_DEBUG and
  BTA_AG_DEBUG flags. These are non-standard flags that must be
  configured at compile time. Instead, debug logs should be gated by
  flags in bt_stack.conf only
* This change enables human readable debug output by default by removing
  these flags, and refactor the string conversion functions

Bug: 70538124
Test: runtest bluetooth, native unit tests
Change-Id: I99fd00016020a9c6b14928906f07bca4f0ea482d