Add support for UnwinderFromPid object.

This object is able to be easily used from other code and can be used
to replace the libbacktrace calls in other parts of the platform.

Also, demangle the function names when calling FormatFrame.

Bug: 120606663

Test: Unit tests pass, debuggerd using this code directly passes unit
Test: tests.
Change-Id: Ifd8cf9bdd89174c1736810711d20e9f37f29b1bf