libbacktrace: make Backtrace::FormatFrameData static.

Backtrace::FormatFrameData doesn't depend on anything from its
instance. Make it static so that callers that don't have a Backtrace*
available can format frames.

Test: mma
Change-Id: Id0ca93bbef19da488305374e58b25c7ca9d0e518
1 file changed