Add flags field to struct android_net_context

This is for passing additional instructions through stages of the
DNS resolution pipeling.

Test: as follows
    - builds, flashes, boots
    - system/netd/tests/runtests.sh passes
Bug: 34953048
Bug: 70694619
Bug: 72345192
Change-Id: I5a86cc5192e6cfa09c375ac9f5810102da3aba20