Send tethered interface callbacks based on interface existence.

Currently, callbacks for tethered interface availability are sent
based on interface link state. This is incorrect:

- It is acceptable and expected to be able to enable tethering on
  an interface that does not yet have link up.
- It doesn't seem useful to disable Ethernet tethering if link
  temporarily goes down.

Instead, only base the callbacks on whether an interface exists
or not.

Bug: 150644681
Test: WIP tests in aosp/1260100 pass
Change-Id: I015219cbd03626c38fb5156d0cb2991ba7d7def7
1 file changed
tree: 3fbc2acd195092d2ea38767a0e016a88518d798d
  1. .gitignore
  2. Android.bp
  3. java/
  4. tests/