Use LE as default transport for GATT

When the application does not specify transport for the GATT connections, use LE transport if LE is supported.

Test: mmm packages/modules/Bluetooth
Flag: com.android.bluetooth.flags.default_gatt_transport
Bug: 372973407
Bug: 376276742

Change-Id: I99a7e18972a1389179f814ae52d083d565a1b3d3