Improve error message when public keys mismatch

Makes it easier to debug tests that are failing because of test APEX
having different public key from the pre-installed APEX.

Test: atest apexservice_test
Test: atest apex_file_test
Test: atest CtsStagedInstallHostTestCases
Change-Id: If44811a51101eac3b6f9cda0e8adc891d01c6b63
1 file changed