Make tether preference controllers extend BasePreferenceController

These controllers extended AbstractPreferenceController, which is not
quite compatible with some of the present framework. Especially for
indexable keys of settings search.

TetherEnabler and AllInOneTetherSettings are refactored based on the
change. Tests are updated.

Bug: 147272749
Test: CodeInspectionTest and other tests of classes touched by this CL
Change-Id: Ic3ad13e735d133c0959a0a309319a6fd5165b015