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: 64133961
Bug: 70694619
Bug: 72345192
Merged-In: I81c49586d675160b66dc2c51b76ad183cc418213
Merged-In: Ie8a8ee5784407d4ec304b3f40a6b234ebeaf6376
Merged-In: Icdec53cf5b210db5ad833dd3b64fa3d8ae74100a
Change-Id: I5a86cc5192e6cfa09c375ac9f5810102da3aba20
(cherry picked from commit 391143e5)