Replace the signing key in test

ApkVerityTestCert.der will shortly be deleted. Replace the key used in
the test and corresponding signature with the existing one (unit_test).

Bug: 277916185
Test: atest VerityUtilsTest
Change-Id: Iaae50b9034eda040d36b06b6b5b53c9c83cc4f5e
6 files changed