//The sample code demonstrates that the headset only reports the battery level.    #define FASTPAIR_ACCOUNT_KEY_SIZE 16    // In the sample code, the size of salt is 2 bytes.   #define SALT_SIZE 2    // 1st byte - Battery level length and type   // 2nd~4th bytes - Battery values   #define BAT_LEVEL_SIZE 3    uint8_t V[FASTPAIR_ACCOUNT_KEY_SIZE + SALT_SIZE + BAT_LEVEL_SIZE + 1] = {0};   int v_index = 0;    // The first 16 bytes are K.   uint8_t K[FASTPAIR_ACCOUNT_KEY_SIZE] = {0};   fastpair_get_account_key_by_index(keyIndex, K);   memcpy(V, K, FASTPAIR_ACCOUNT_KEY_SIZE);   v_index = v_index + FASTPAIR_ACCOUNT_KEY_SIZE;    // The next byte is the Salt.   uint8_t randomSalt = (uint8_t)rand();   V[v_index] = randomSalt;   v_index = v_index + SALT_SIZE;    // The remaining bytes are the battery information (from s + 1 to s + 4 including the length and type bytes).    uint8_t battery_level_len = 0;   uint8_t battery_level[BAT_LEVEL_SIZE] = {0};    fastpair_get_bat_level(&battery_level_len, battery_level);    // type = 0b0011 (show UI indication) or 0b0100 (hide UI indication)   V[v_index] = (battery_level_len << 4 | (is_show_ui ? 0x3 : 0x4));   v_index = v_index + 1;    for (int idx = 0; idx < battery_level_len; idx++) {           V[v_index++] = battery_level[idx];   }