Don't collect useless stack frames; do demangle C++ symbols.

Don't collect useless stack frames; do demangle C++ symbols.

Previously, we'd collect every stack frame and then throw some away
when we came to log them. This meant that stack traces were effectively
shorter than the buffers that had been allocated for them. This patch
only stores frames we'll actually output.

Also dynamically call the C++ demangler so we don't have to try to
read mangled names. Because no one knows the mangling of operator new[]
for int arrays off the top of their head.

Bug: 7291287
Change-Id: I42b022fd7cd61675d05171de4c3b2704d058ef2a