For emulator builds preopt system apps even if doing eng build

Following change disabled preopt for system apps when doing eng build:
    4df56578

    Build: Only preopt boot images in eng builds

    As a middle way between full preopt/high performance/long builds,
    and no preopt/low performance/fast turnaround, preopt only the
    boot image in eng builds.

    I4a2692f3ce84823cd40c6b7d672fd73257739ef8

This works well for devices, but first boot on emulator takes 10+mins.
Bypass the change by forcing preopt inside the BoardConfig.

Change-Id: I58d100cd65d2a09b644a90d91261102aab31fcbb