Previously, the test mistakenly configured BootJars which meant the
test was testing not just the propagation of paths to files retrieved
from the prebuilt apex to the java_import module but also the process
of finding appropriate boot jars.
It also set "prefer: true" on the prebuilt_apex instead of the
java_import when checking to make sure that it still found the correct
module when both prebuilt and source were available and the prebuilt
was preferred.
Bug: 171061220
Test: m nothing
Change-Id: I4f2fa7ea372b14d4b169cdc78bdeb1ca4ac4dff8