Make android_logger_set_prune_list() sane

The current version requires callers to supply a string with 32 extra
bytes for liblog to internally prepend "setPruneList ", and to have
enough space to parse logd's return string.  That is an unacceptable
requirement on callers.

This change removes that requirement by having liblog allocate the
needed std::string in any case.

It also stops writing back the 'success' or 'Invalid' string to the
caller's buffer, since that is redundant as well.

Test: changing prune settings works.
Change-Id: Ic0f03a229f0b9a77d03adcb91288370c3bd42903