Fix some easy cases of __ANDROID__ versus __BIONIC__.

We need to make a clearer distinction for bionic on the host. This patch
doesn't fully address things like "should host bionic try to talk to netd?"
for now, but is a step in the right direction.

Bug: http://b/31559095
Test: bionic tests.
Change-Id: I49812f8b75d9d78c4fd8a8ddf5df1201d63771d6