Implemented control of "export only" feature in dexdump.

Rationale:
Export only is now under explicit control of a new -e flag, and
can be used to control both -l xml and -l plain. The result is
that, by default, xml output now shows all items (old behavior
can be forced with -e -l xml). This solution feels a bit cleaner
than adding a flag to override export only when -l xml is given
(old dexdump forced this tight relation).

Bug: 25673756 (feature request).

Change-Id: I7cfd48bf91195d2a81d134936719d267fda399f5