lowpan: Remove libandroid_net_lowpan from platform/frameworks/base

This change removes the Android.mk file that created the target
libandroid_net_lowpan from platform/frameworks/base.

This native library has moved to frameworks/opt/net/lowpan via the
following change id:

    I3dc7b9ca3b33b2ee5f92c8f76c28710997fb931d

Bug: b/63707448 b/63708348
Test: Confirmed with unit tests from change id
      I41d590b1e77dc41873c4b9e9bf1b7f1bf859f74e
Change-Id: Ifbfe75dbfbfc0ac2c02124dcac151a9caf6b342a