Ignore System Server dynamic dex files during A/B preopt

System Server dex files has a special dexopt path while the
OTA service uses a PackageDexOptimizer backdoor which breaks
the assumptions.

Ignore any file loaded by system server until we update the
OTA service to use the proper PMS dexopt paths.

Test: adb shell; /system/bin/otapreopt_script 1
Bug: 160735835
Change-Id: Ic3d0a3127862c99c7e29ce552f6d89c248df1d0c