ANDROID: Repurpose a reserved slot in ipv6_devconf for backports

This patch repurposes a ANDROID_KABI_RESERVE slot used for LTS backports
for feature backports. Slot 4 is repurposed as parts of slot 1 are
already used for accept_ra_min_lft on some branches.

Bug: 315069348
Signed-off-by: Patrick Rohr <prohr@google.com>
Change-Id: I19b9dfc16d891fb6fe48ec4379c6fa3dcb6adf89