Allow precompiled sepolicy w/o system_ext/product

Some devices might not have system_ext or product partitions. But init
has been refusing to use precompiled sepolicy because init always checks
system / system_ext / product hashes, regardless of existence. This
makes system_ext and product optional, so hash check can be skipped for
non-existing partitions. Of course system is always checked.

Bug: 181640066
Test: boot microdroid and cuttlefish, see precompiled sepolicy works
Change-Id: I32c296fffd894c27097e8b4e10ade977a21d61ab