Linker namespace configuration for the Resolv APEX.

Support netd to load resolv Apex.
Switch namespaces when switching library paths between
/system and the APEX, so that internal library dependencies in both
locations are loaded from their own directory.

Bug: 119527674
Test: make; flash; lsof -p $(pidof netd)
Test: 1. manual test datacall/wifi work
      2. manual test tethering work
      3. system/netd/tests/runtests.sh
Change-Id: I3f69e85f2f529636f0ef29a2d9d71ad582c46dfb