Fix javax.crypto.AEADBadTagException Crash by Adding a Secret Key String Resource

[yaxarat]

This PR addresses https://github.com/googlecodelabs/biometric-login/issues/2 by providing the correct string resource to secretKeyName introduced in section 01 of the code lab.

Otherwise, the app will crash on decryption with javax.crypto.AEADBadTagException.

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[yaxarat]

@googlebot I signed it!