Fix CoexUtils.getOffsetChannel not counting by channel step size

CoexUtils.getOffsetChannel should return channels counting by the given
channel step size. This fixes an issue where a channel step size of 4
(for 5GHz wifi 20MHz channels) would be ignored and channels in-between
the step size would be returned, resulting in invalid 5GHz channel
numbers.

Bug: 191267443
Test: atest CoexUtilsTest
Change-Id: I6f98354d2613fbd65e8fa1a056429522b9e108ca
2 files changed