com.android.tethering: set min_sdk_version 30

Tethering is updatable apex module that was launched in R, hence it and
all it dependencies should specify min_sdk_version <= 30.

Test: m
Bug: 171668006
Bug: 171330443
Change-Id: Ic91cf96dda6419d1038b0329b920f9cd24482aef
Exempt-From-Owner-Approval: Mark is owner and gave +2