FenceTime: Fix undefined behavior

std::unordered_map::erase() invalidates iterators to the erased
elements. Using erase() inside a range based for loop can lead to
undefined behavior, because the loop holds the same iterator that are
invalidated.

Fix the problem by using an interator directly and incrementng only when
it makes sense.

This was found by surfaceflinger_frametracer_fuzzer running with asan
(not hwasan!).

TESTED=only fuzzer

Bug: 307601836
Change-Id: Id99feaec21300dbd55d35acba67801b2483dd144