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/libbacktrace/UnwindMap.cpp b/libbacktrace/UnwindMap.cpp
index af79562..0b8232b 100644
--- a/libbacktrace/UnwindMap.cpp
+++ b/libbacktrace/UnwindMap.cpp
@@ -57,7 +57,7 @@
     map.start = unw_map.start;
     map.end = unw_map.end;
     map.offset = unw_map.offset;
-    map.load_base = unw_map.load_base;
+    map.load_bias = unw_map.load_base;
     map.flags = unw_map.flags;
     map.name = unw_map.path;
 
@@ -106,7 +106,7 @@
       map.start = unw_map.start;
       map.end = unw_map.end;
       map.offset = unw_map.offset;
-      map.load_base = unw_map.load_base;
+      map.load_bias = unw_map.load_base;
       map.flags = unw_map.flags;
       map.name = unw_map.path;