Handle boot jars on /system_ext correctly.

When Soong looks for boot jars among all modules, it applies certain
constraints to each module that looks like a boot jar (e.g. that it
comes from the right apex or platform). Previously these constraints did
not handle boot jars on /system_ext correctly (they were handled like
apex jars, while they should be handled like platform jars).

Bug: 154976937
Test: m nothing (the modified Soong test would fail previously)
Change-Id: I0746a2fd276ab5ef0400340c5b61cf26c2570e5a