Add IPsec tunnel migration feature flag

This patch adds a feature flag for IPsec tunnel migration. This
implies XFRM_MIGRATE enabled in the kernel.

Bug: 169169973
Test: make
Test: verified on sunfish-eng: added the xml in device.mk,
      built and flashed the phone, the xml showed
      in "adb shell pm list features"
Change-Id: I1d3d6739e0059bbb522b6ce9e3c410ff21089e66