Improve hidden API checking in TestBootDexJarsFromSourcesAndPrebuilts

Previously, the test only checked how the intermediate index file was
created from class jars. This change also checks how the monolithic
index file was created from the intermediate index file and the index
files generated from each bootclasspath_fragment.

At the moment the checks reveal that the monolithic index file is
created solely from the intermediate index file. However, a follow up
change to Soong will alter the behavior of the test.

Bug: 194063708
Test: m nothing
Change-Id: I343712d7d17ab0a07f5b32226321fc6815847e1f