Guard Android DNS resolver hack behind !cgo

[dyhkwong]

When v2ray-core is compiled with CGO, this hack is not needed. This also makes it problematic to use v2ray-core as a native library on Android, which almost comes with CGO enabled (compiled with NDK).

Example: I need to remove the hard-coded DNS for v2ray-plugin-android to make it work. https://github.com/ExclaveNetwork/v2ray-plugin-android/releases/tag/v1.3.4

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.29%. Comparing base (63c44fd) to head (8be9b91).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3755   +/-   ##
=======================================
  Coverage   25.28%   25.29%           
=======================================
  Files         905      905           
  Lines       56382    56384    +2     
=======================================
+ Hits        14255    14260    +5     
+ Misses      40320    40315    -5     
- Partials     1807     1809    +2     

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