Extract default image specific logic from buildBootImage

Previously, buildBootImage had to be called for the default boot image
config before the art boot image config as it created some rules which
were specific to default boot image. The creation of these rules was
encapsulated in Config().Once(..) to ensure that they were only ever
created once.

This change extracts that functionality out of buildBootImage to the
calling method which means that they are only called once and so no
longer need protecting against being called again.

Bug: 177892522
Test: m droid
Change-Id: I464477de1a08df15e577873a9accf7db2bc088d1