Exclude the system server from background dexopt and remove the code

that optimizes it.

The system server can only be compiled and signed by odrefresh in early
boot, so it is no use capturing dex load events for it.

This complements commit 9789a45e.

Test: boot
Bug: 254043366
Change-Id: Idda7957da72d72ddccccf95580d1bd85b520358f