android_system_image only packages "system" items

android_system_image filers packaging items installed outside "system"
partition.

Some packaging items install related items to different partitions but
putting them altogether to android_system_image doesn't make sense.
(android_system_image is suppposed to be "system" partition)

To be specific, this filters out "apex" partition items.  "apex"
partition is used by APEX installation to install APEX contents to paths
similar to activated paths on device so that symbol lookup works well
with APEX contents.

Bug: 225121718
Test: atest MicrodroidHostTestCases
Test: debugfs <intermediate>/microdroid.img -R 'ls system'
  shows no "com.android.runtime"
Change-Id: Ibc3d85ead2fda99e231132ce8ab9ccf1cc9317b7