Add tethering offload HAL call via JNI

Additionally, try to locate java Control interface.

Test: as follows
    - built
    - flashed
    - booted
    - OffloadController log messages observed, but only ever
      "not enabled" messages (needs a supporting implementation)
Bug: 29337859
Bug: 32163131
Bug: 34361337
Change-Id: I5251d05f2d2fd732a33a8955a6c346b3a2401e46