Fix TestAarImportProducesJniPackages/aar-no-jni

TestAarImportProducesJniPackages was attempting to assert that an
aar_import module always had a JniPackageProvider, but was instead
asserting that the return type was always a JniPackageInfo.  Since
ModuleProvider returned the zero value of JniPackageInfo even when
there was no provider that was always true.

Fix aar_import to always set JniPackageProvider.

Test: TestAarImportProducesJniPackages
Change-Id: Idaf106e14373490d4eb807892e174aaaf094bbcf