Updates and fixes to android.net.lowpan

This commit is an incremental improvement to the LoWPAN support API.

Bug: b/33073713
Test: Successfully ran unit tests from Ib3750be5052bf1a90bf871756e9121b047d3871f
Change-Id: I7ca6374b6a7135411eadf631bd7d84d7dc008203
9 files changed