Flag bta_dm_disc into new and legacy code

Keep bta_dm_disc_legacy with legacy code without refactor

Bug: 335732980
Bug: 330675788
Flag: com.android.bluetooth.flags.separate_service_and_device_discovery
Test: mmm packages/modules/Bluetooth
Change-Id: I843c499b5c2d71bb12e8a28592221b1516873802