base: tag unique_fd::reset as reinitializing for clang-tidy.

Appease clang-tidy by marking reset() as a method that reinitializes
after moving out of a unique_fd.

Unfortunately, there isn't an attribute that let us mark the type as
being safe to use after move in general, which means that moving out of
a unique_fd and then calling get() on it will still be frowned upon.

clang-tidy has a hard-coded list of standard container types that
are safe to use after move, but doesn't provide a way to mark custom
types as satisfying this condition.

Bug: http://b/150959261
Test: reverted the change to unique_fd.h and the test failed
Change-Id: Ide73d7caa4cd2b192018f111059d696dca4de987