gta4xl-common: Add feature flag for FEATURE_IPSEC_TUNNELS

This commit adds the IPSEC_TUNNELS feature flag, with XFRM Interfaces
used as the implementation of choice.

Bug: 117183273
Bug: 137629222
Test: Compiles, passes tunnel-mode CTS tests
Test: adb shell pm list features
Change-Id: I19c598947525783850b29d149db85db89f8b347e
1 file changed