Move resolver public headers to separate include path

Additionally rename functions and structs to eliminate those ugly
underscores from the public API (with the notable exception of
__res_params, which will come in its own separate change to keep this
one from growing too large).

Test: build, flash, then 'atest netd_integration_test'
Change-Id: I7e9b0ee6cdbec889e9c35b5a17b5daa65533686e