HCI: Use enum for LeExtendedAdvertisingResponse.event_type

Use a bitmask enum for the LeExtendedAdvertisingResponse.event_type
field. This simplifies parsing and enables duplicated bitmask
definitions to be deleted. This is not intended to change
behavior.

Bug: 315496838
Test: atest LeScanningManagerTest
Test: atest LeScanningReassemblerTest
Flag: EXEMPT no logic change, just moving definitions
Change-Id: Ie4609653cfacb14332181f2bb4dd24957eccadd8