Replace isTetheringWithSoftApConfig flag with B SDK check

isTetheringWithSoftApConfig flag is finalized, replace it with a simple
SdkLevel.isAtLeastB() check. This also fixes any issues with reading the
flag during boot.

Bug: 216524590
Test: covered by existing tests
Change-Id: I6c2717a68fa4f3c664c9948ec19b246db5afdb8b
4 files changed