Move from libbacktrace to libunwindstack.

Use the new AndroidUnwinder object to replace the libbacktrace version.

Bug: 120606663

Test: Run unit tests.
Test: Force a call to CallStack::log() and verify the backtrace is
Test: reasonable.
Change-Id: I73d909821a33f716d7b0e0997d43c6ab11c7c4f9