gd::hal::snoop_logger Return proper empty dumpsys structure

Dumpsys entry does not use flatbuffers directly but only
as mechanism to indicate when to write snoop log

Extract `EmptyDumpsysDataFinisher` into separate compilation unit

Bug: 314828950
Test: atest bluetooth_test_gd_unit
Test: manual adb shell dumpsys
Flag: EXEMPT, unchanged logic mechanical movement between compilation units

Change-Id: Icf2d902ddd27278fdd4517eda18c1ad613b91d44