Fix problems with TestPrebuiltExportDexImplementationJars

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