DoH: Refactor FFI

* Factor out constants to level enum conversion
* Document property that doh_init_logger will have no effect on future calls
* Change `doh_set_log_level` to default to `Error` logging on an invalid
  value rather than disabling all logging. This matches the behavior of
  `doh_init_logger`.
* Prefix constants with `DOH_`. `cbindgen`-exported constants are not in
  a namespace; they are raw C constants. Prepending the library name
  will help avoid future collisions.
* `android_logger::init_once` explicitly states in the documentation that
  calling it more than once is safe, and will simply not change the logger
  on future calls.

Bug: 202081046
Change-Id: I91a8a7f40363ee2dcf6211fb595e1c6e7f6084a8