Split TEST_MAPPING for resolv/ to its own directory

This is a preparation step for splitting the resolver to a separate
repository. Running 'atest' without arguments in either system/netd/ or
system/netd/resolv/ executes the same tests.

The 'imports' section is documented here:
  https://source.android.com/compatibility/tests/development/test-mapping

Test: atest
Change-Id: I2470f41c344dcf448bbaa35f4f0d5e9f0ce9a7a3