Refactor 1.0 TetheroffloadControl VTS

This is a pure no-op refactoring that split the implementation
into several files. So the follow-up tests might be able to
re-use them to prevent from code duplication.

Test: atest VtsHalTetheroffloadControlV1_0TargetTest

Bug: 149467454
Bug: 170699770
Bug: 170179169
Change-Id: Icda34e3eb4d4ccb9f4564e23a61bd71323ca9bb3