Update summary for WifiTetherDisablePreference accordingly

The summary of this preference is depenent on other two preferences for
USB tether and Bluetooth tether. The summary is updated accordingly.

Bug: 146818850
Test: CodeInspectionTest, WifiTetherDisablePreferenceControllerTest,
manually test.

Change-Id: I366d10a43814ea0d7def466b1976928f17875029
3 files changed