Make NetworkStack actually build in an unbundled setup.

The Java code was already using sdk_version: "current", but the
JNI library did not have an sdk_version attribute, and had
forbidden dependencies on internal libraries such as libcutils,
liblog and libnativehelper.

This CL fixes things as follows:
- Use the NDK log functions, and switch to the NDK liblog.
- Switch to the NDK libnativehelper.
- Drop the dependencies on libcutils and libpcap, which were
  unused.
- Use the libc++ STL, and set the sdk_version of the library to
  current.

Bug: 131703417
Test: m
Test: installed NetworkStack APK via "adb install --staged" results in working networking
Test: builds, boots, wifi works
Test: atest FrameworksNetTests NetworkStackTests
Change-Id: I1a7bd7fe2809b90a66073d8c5d5dcdff646c4286