Refine TetherSettings to avoid activity leaks

- Get system service by application context instead of fragment context to avoid the context being occupied

- Use the application main executor instead of creating new executor locally to avoid the executor being occupied

Bug: 246531382
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=TetherSettingsTest
atest -c TetheringHelperTest

Change-Id: I0de5e9c2bedfc8a224b3f8306224542212c9fcf5