Improve key_missing_classic_device handling

Just like with btm_io_capabilities_rsp, in *_req handler we must check if device is already encrypted.

Bug: 333634398
Bug: 376765985
Test: mma -j32;
Test: OPP/SR/GOEP/BC/BV-01-I
Flag: com.android.bluetooth.flags.key_missing_classic_device
Change-Id: Ib572d92baedb8cad62e0482349a48d9dd310089b