Refactor DexFile::DecodeDebugInfo.

Split the method into two - one for locals and one for positions.
All uses of the method request only one of the two and it makes the
code slightly cleaner. The position variant requires fewer parameters.

Expose additional line table information which was previously ignored
by the decode method (prologue, epilogue, source file).

Change-Id: Idf8ba98fa58ea0d2103932b5cc0af81365885107