Add return code KEY_ALREADY exists

In preparation to the async keystore interface we change the semantics
form unconditionally overwriting existing keys to reporting that the key
exists. For compatibility we reimplement the same semantic in the
calling code.

Bug: 111443219
Test: KeyStore CTS test
Change-Id: I1fa5428fa7ada97d5068778cd4590593c992554d
1 file changed