Do not throw on call to isTetheringSupported w/o permission

...just return false instead. This will change in P.

Test: Made an app to test this. Made sure it doesn't have
Test: the required permission. Checked it crashes with
Test: SecurityException without this change. Checked it
Test: doesn't with it.

Bug: 65404184

Change-Id: Id20d3c240ec5d70d085e0366b92ab3a514f3e7c8