Update SoftApState for API council feedback

Update SoftApState with
1) Throw IllegalStateException if getFailureReason() is called when not
   in failure state.
2) Clarify the compatibility story between the new onStateChanged and
   the old.

Bug: 329899277
Test: atest SoftApStateTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ad2b55a9469eb450c1184769d9553a51a55f6f93)
Merged-In: Ib12360af81ef2df6125e58daf28ad014517cea8b
Change-Id: Ib12360af81ef2df6125e58daf28ad014517cea8b