ClatdController::ClatdTracker - rename 'dst' to 'pfx96'

Whether something is dst or src depends on direction,
but 96-bit prefix is always meaningful for clat translation.

Test: atest netd_unit_test
Bug: 65674744
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Id7d2177d5282709db865b2a86e6c9b7cb6778e80