Add a soong var that will be used to enable/disabe rust relevant targets am:...

Add a soong var that will be used to enable/disabe rust relevant targets am: 5b7787ec am: 5e6a2f7e am: 7892e66a

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/13929889

Change-Id: I931b15cf1a18e0ad8f0c20dd77e7630927b7ee2e