Factor out duplicate code for parsing gap data

This change is intended to be used to factor out
dup code for parsing GapData in StartAdvertisingSet
and make it easier to be tested.

Backport of Ia39886c415218353b6f9d59d7d3f6d1160477d6c

Bug: 296291440
Test: atest net_test_main_shim
Merged-In: Ia39886c415218353b6f9d59d7d3f6d1160477d6c
Change-Id: Ia39886c415218353b6f9d59d7d3f6d1160477d6c