cronet: add defaults to enable / disable Cronet in framework-tethering am: 3b9ba6acdf

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/2374722

Change-Id: If7d0fa79a53a753362a63f6c3c8769e03439f60f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>