Clean up old code that only applied to upgrades from Android older than N.

This means completely removing the dexopt progress dialog, which was
only shown on such an upgrade. The fstrim message was blocked on that
dialog, which means it's practically never shown. This CL changes it to
show regardless.

Test: Boot
Bug: 251903639
Change-Id: Iba515866a085c0abb2e7701b95249a5863da2bc5