Run LowpanManager on BackgroundThread

The ConnectivityThread class is being separated into a specific
connectivity JAR; transport-specific managers like LowpanManager should
not be sharing the ConnectivityThread.

As callbacks from ILowpanManager / ILowpanManagerListener already do not
have any ordering guarantee with ConnectivityManager callbacks, the
impact of this change should be minimal.

LowpanManager is unused in AOSP, and not part of the API.

Bug: 174436414
Test: m
Change-Id: I23483ed7c4a6c5283b365430a3e503a0dd86c2cb