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

Original change: https://android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/1642562

Change-Id: I179b2c564c11f3244d4e7997e4a04e4120374634