Use current pid not tid to create a backtrace map.

The BacktraceMap::Create() function assumes that the passed in
parameter is the current pid not tid. If a tid is passed in, then
the logic that creates the local map in libunwind is not used
and can bypass all of the special handling of local maps. The result
can be crashes if this map data gets out of sync with the real maps
found in the process.

Bug: 26676205
Bug: https://code.google.com/p/android/issues/detail?id=199219
Change-Id: Ibab9f7a7b99648a2cdaa185958bf6ffc368079ce