lowpan: AIDL refactor to no longer use property design pattern

After doing a considerable amount of thinking on the subject,
I decided that attempting to directly port wpantund's property-based
API over to Binder was a mistake, so this commit contains the required
changes to abandon that approach and go with a more traditional
Binder interface.

Bug: b/63708348
Change-Id: I685cd066fabe8470ef4bb456aea7d3bb515c3969
Test: Compiled and ran unit tests, manually confirmed lowpanctl
      and lowpan-service (with related changes) appeared to be
      working properly.