eatt: Establish EATT after encryption is done.

As per specification EATT channel shall be encrypted and this patch
makes this happen.

However for some PTS test cases, we still would like to create L2CAP
channels without encryption and this patch also allows to do that with
flag PTS_ConnectEattUnencrypted

Bug: 236944522
Test: PTS L2CAP/ECFC/BV-32-C
Test: atest BluetoothInstrumentationTests
Test: atest net_test_eatt
Tag: #feature
Change-Id: I98d7721f3733756e2c47bc7acd6f36d571b0eac0