Move stack/l2cap/::l2cu_get_next_channel_in_rr

From: stack/l2cap/l2c_utils
Into: stack/l2cap/l2c_link

Towards proper interfaces

Bug: 163134718
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I6bd6f659d312c893564de7d3c07d429d38da76a8