FTL: Make StaticVector::replace infallible

std::move from a temporary to avoid bailing out when the vector is full,
with the caveat that replacing at end() is no longer valid.

Also, disable iterator constructor for ambiguous types, and document
preconditions/postconditions for all functions.

Bug: 160012986
Test: ftl_test
Change-Id: I78397517ed11098067fff1cb04f735b29756c9cf