keystore: rephrase the comment to make the usage more clear.

#endif

/* This function is provided for native components to get values from keystore.

 * Users are required to link against libcutils. The lengths of keys and values

 * are limited to KEYSTORE_MESSAGE_SIZE. This function returns the length of

 * the requested value or -1 if something goes wrong. */

 * Users are required to link against libcutils. Keys are values are 8-bit safe.

 * The first two arguments are the key and its length. The third argument

 * specifies the buffer to store the retrieved value, which must be an array of

 * KEYSTORE_MESSAGE_SIZE bytes. This function returns the length of the value or

 * -1 if an error happens. */

static int keystore_get(const char *key, int length, char *value)

{

    uint8_t bytes[2] = {length >> 8, length};