Remove hashtree from builtin APEXes

Builtin APEXes (/system, ...) don't need hashtree.
So if we remove hashtree from builtin APEXes we can save more space in
/system.

However, this change only affects to the apex bundles which are built
from source, not from prebuilts.
Removing hashtree from prebuilts will be handled in a follow CL.

Bug: 139957269
Test: m
Test: see if /system/apex/*.apex have hashtree
Change-Id: Id755382b2153bf6bbaf2c480f3bbb91b6f62a564