Require actual certificates for testNoAddedCertificates.

Until now, this test required only SHA-1 fingerprints of OEM-added
certificates to be present. To ensure more transparency and future
extensibility this CL makes this test require full contents of
certificates.

P.S. The base class of this test was changed from AndroidTestCase to
InstrumentationTestCase because it now needs to access assets from
the test package. Assets contain permitted OEM-added certificates.

Bug: 11483665
Change-Id: I1a4ab0e4ad516102ec5f1fce855be5c2ba988666