Add an asynchronous dexopt method.

This method is to be used by the package manager, to move dexopt out of
the package manager handler.

This CL also deletes some out-of-date comments.

Bug: 388159696
Change-Id: I6ca0a17469dea6c22a3c8889bbb819b6b966401a
Test: m
Flag: EXEMPT refactor