dexdump: Add dump string_ids capability (-s)

It can useful to inspect dex markers. These are contained in the
string_ids section but not referenced by anything.

This CL adds options -s to dump all strings in a dex file without
accounting if they are referenced or not.

Test: dexdump_test.cc
Bug: 283737440
Change-Id: If1b384638f3164728e2b90d6796e0397e536ad86