Set CATEGORY_UNDEFINED as default category

It's possible that there is no tag <application> in the mainfest.
Therefore, category wouldn't be set to CATEGORY_UNDEFINED properly.
Assign the proper default value for this case.

Fix: 207456733
Test: atest AndroidPackageInfoFlagBehaviorTest
Test: atest AndroidPackageParsingEquivalenceTest
Change-Id: I5fb82fea5ccdf216b5825c0c3a254d8be3666c04