eatt: Accept incoming Eatt connection request for uknown devices

Android checks Server Supported Feature characteristic before it allows
to create EATT channel. However for LE it is not necessary to test this
characteristic, and if remote device sends connection request it is OK
to assume EATT is supported and add it to known devices.

Bug: 235195694
Test: atest BluetoothInstrumentationTests
Tag: #feature

Change-Id: I2651e431feab4603a10a05334458009525948cf4
(cherry picked from commit cf102edb)