Use INetd Constant for IpSec Interface Prefix

Convert to using a constant in INetd to ensure
that there is a consistent tunnel prefix  between
Java and native code.

Bug: 74560705
Test: atest FramworksNetTest; atest CtsNetTestCases
Change-Id: Ida233aac2e6c6b26567463964e0ebac9d52eff1e
(cherry picked from commit 649df5e0)