MockDrmCryptoPlugin: Don't truncate log output

A function argument which is an array with a fixed size is not
actually a fixed size array, but rather just a pointer.  Thus,
when we were using sizeof() for 'key' and 'iv', we were mostly
likely only logging the first 4 or 8 bytes of the array, and
not all 16.

We change this code so we're at least passing the correct size
to the arrayToString method, and thus logging all the bytes.

This API is still fragile, as there is nothing which assures
callers to decrypt are passing in arrays which are exactly 16
bytes.  However, this API is equivalent to before, so we aren't
introducing any compatibility issues.

The compiler gave a warning about this.

Test: Compiled and confirmed there's no longer this compiler warning.
Change-Id: Ie3a32eef0882ea95bccc3e22ffcab7336779bd4c