Cleanup the libbacktrace interface a bit.

- Change the field name load_base to load_bias (which is what it really is).
- Add a rel_pc field so that callers do not need to compute it themselves.
- Remove the BacktraceMap::GetRelativePc() since nobody should need to
  compute this themselves.

Bug: 23762183

Test: Compiles and unit tests pass (debuggerd, libbacktrace).
Change-Id: I2cb579767120adf08c407a58f3c487ee3f2b45fc