Fixing engineGetCertificateChain exception

If a certificate is self signed, then currently KeyStore will still
attempt to find the CA certificate. When it obviously fails to find it,
a key not found exception is propagated up and thrown. This CL
suppresses that exception, as it seems to exclusively be thrown in this
condition, which is WAI. Having the stack trace show up can be very
misleading to developers.

Test: atest cts/tests/tests/keystore/src/android/keystore/cts
Change-Id: I192f54d3d8355c183e830ab09314932e8800f7ed