Clean up TetheringRequest#toString()

Clean up TetheringRequest#toString() by printing the TetheringType and
ConnectivityScope in a human-readable form and only printing the values
of fields that have been set.

Bug: 216524590
Test: atest TetheringManagerTest
Change-Id: Ie2c35190a09605e3018b9d2d1f1d00cdc7bbb977
2 files changed