Add a BARRING_TYPE_UNKNOWN for Unreported Barring

For vendors that do not support barring info reports,
add an additional BARRING_TYPE to express the difference
between unbarred and unknown barring status.

Bug: 143226991
Test: make update-api && make && atest BarringInfoTest
Change-Id: Ie9314c5612536bf181a3c0e52876b1804ed7cce2