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
diff --git a/debuggerd/libdebuggerd/tombstone.cpp b/debuggerd/libdebuggerd/tombstone.cpp
index 996d714..0113131 100644
--- a/debuggerd/libdebuggerd/tombstone.cpp
+++ b/debuggerd/libdebuggerd/tombstone.cpp
@@ -466,11 +466,11 @@
         line += " (BuildId: " + build_id + ")";
       }
     }
-    if (it->load_base != 0) {
+    if (it->load_bias != 0) {
       if (space_needed) {
         line += ' ';
       }
-      line += StringPrintf(" (load base 0x%" PRIxPTR ")", it->load_base);
+      line += StringPrintf(" (load bias 0x%" PRIxPTR ")", it->load_bias);
     }
     _LOG(log, logtype::MAPS, "%s\n", line.c_str());
   }