Better error handling when NetworkManagementService#untetherInterface() fails.

Additionally, catch ServiceSpecificException when making netd binder calls.

Test:
  Flipped back and forth between USB and wifi tethering and no longer
  some of the unrecoverable errors.

Bug: 30882741
Change-Id: I470b7cfdaaa82013db35b500a5df10dcfaad2d8c