Bluetooth: ATT: Split BT_ATT_MTU for UATT/EATT

This is a refactor. There is no behavioral change because
`BT_LOCAL_ATT_MTU_UATT` is `BT_LOCAL_ATT_MTU_EATT + 2` is
`BT_ATT_BUF_SIZE` is the old `BT_ATT_MTU`.

The old `BT_ATT_MTU` was wrong for EATT bearers. EATT MTU is two bytes
less because of ECRED overhead.

Instead of the old `BT_ATT_MTU`, we define one for each bearer type. We
also define the max of them to use as a convenience for allocating
buffers that fit either.

To avoid confusion, 'LOCAL' has been added to the name. This is to
differentiate it from what the spec calls 'ATT MTU', which is a
negotiated property. (It is the minimum of the two side's local ATT
MTU.)

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>