gatt: Fix handling MTU Exchange request

With this patch Android will always do only one MTU Exchange request as
per specification. For this reason Android always uses
GATT_MAX_MTU_SIZE as this is what easly GATT can handle in the Bluetooth
Stack.

This is done so the single application request cannot block other
applications from using bigger MTU than the one requested by the first
application.

For backward compatibility, the for Data Length Extension the MTU from
the User is used and not the one which was internally requested by the GATT

Bug: 257250188
Bug: 239776509
Test: atest BluetoothInstrumentationTests
Test: manual testing
Tag: #feature
Change-Id: I157d40040b53cd4595403d99e58e105709099684