Move map data into backtrace data proper.

The backtrace structure used to include a pointer to a backtrace_map_t
that represented the map data for a particular pc. This introduced a
race condition where the pointer could be discarded, but the backtrace
structure still contained a pointer to garbage memory. Now all of the map
information is right in the structure.

Bug: 19028453
Change-Id: If7088a73f3c6bf1f3bc8cdd2bb4b62e7cab831c0