Allow opening tethering when data saver ON

When data saver enabled, tethering would be OFF.
Currently settings would not allow user to turning tethering back.
After aosp/1181583 is merged, user can turn tethering back without
turning data saver OFF.

Bug: 145711175
Bug: 142374233
Test: atest TetheringTests
      OFF/ON tethering when data saver ON

Change-Id: I59e662ba771a563f5f1766ba29e05246b8280220