Define tCONN_ID for GATT Connection ID

Introduce tCONN_ID as a type for
GATT connection IDs used internally
by the native stack.

Althogh this change itself doesn't do anything,
this is a preparatory step for expanding
tGATT_IF to uint16_t and tCONN_ID to uint32_t.

Rust stack cleanup will follow in a subsequent CL.

Flag: EXEMPT, mechanical refactor
Bug: 273561907
Test: atest BluetoothInstrumentationTests
Change-Id: I307fbb0815f3f3c31c80e75c12c4223237099adc