SMP: Calculate LTK on BR SMP begin

With the original code, SMP central device calculates LTK after
receiveing keys from peripheral, but this should be done before
key distribution.

Tag: #compatibility
Bug: 198554515
Test: atest BluetoothInstrumentationTests
Change-Id: I2127f7210374ad2afaaacee31b3916c7ce3e2597