Change 75830fb836 to fix _nres
initialization to be thread safe accidentally introduced a behavior
change whereby res_init() became a no-op. It also failed to remove all
direct accesses to _nres.
Move the file over to C++ so we can let RAII ensure we're always holding
a lock while using the global state, make all callers access the global
state via this class, and restore the previous behavior of res_init().
Test: atest DnsResolverTest
Bug: 166235340
Change-Id: Ib390a7eac063bc0ff5eeba755e8c74ef1383004e
