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This CL also deletes some out-of-date comments.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
Test: m
Flag: EXEMPT refactor
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:31966938245cf259dba656709cfb73a6219e7e62)
Merged-In: I6ca0a17469dea6c22a3c8889bbb819b6b966401a
Change-Id: I6ca0a17469dea6c22a3c8889bbb819b6b966401a