android/gatt: Fix MTU exchange
android/gatt: Fix MTU exchange

This fix using invalid MTU on LE link. Previous attempt to fix this
"android/gatt: Fix initial setting of MTU" was not correct since
on LE default MTU is always 23 and we are not allowed to send bigger
requests before MTU exchange was performed.

This was affecting multiple PTS GATT test cases.

pkarasev3/bluez,silent-snowman/bluez,mapfau/bluez,silent-snowman/bluez,ComputeCycles/bluez,mapfau/bluez,mapfau/bluez,ComputeCycles/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,silent-snowman/bluez,pkarasev3/bluez,mapfau/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez