Make libnl available for NetworkStack and tethering module

Introduce `libnl_static` to allow the `NetworkStack` module to
statically link `libnl`. The `NetworkStack` module is an APK, not an
Apex, which requires static linking for its dependencies. Additionally,
set `sdk_version` on the target. This ensures an NDK variant of the
library is created, enabling `libnetworkstackutilsjni` to link
against it successfully.

This change also add com.android.tethering to apex_available list

Test: TH
Flag: EXEMPT PURE_REFACTORY
Change-Id: I5df1e3eff76d1f952b844cf86312eb109bb357db