logd: remove min heap in SerializedFlushToState

There was a bug in SerializedFlushToState::Prune() caused by an access
to a SerializedLogEntry raw pointer as a member of a MinHeapElement,
which was deleted earlier in the function.

Instead of just fixing the order of the access and the deletion, I
sought out to remove the raw pointer entirely.  In doing so, I noticed
that the min heap doesn't provide significant benefit, since we'll
only ever have 8 log buffers so scalability is not an issue.

Therefore this change removes the min heap entirely and uses the
existing log_position_ and logs_needed_from_next_position_ members to
keep track of which are the next unread logs.

It also adds a smoke test for SerializedFlushToState::Prune() and
additional CHECK() statements to help prevent future errors.

Bug: 168869299
Test: unit tests
Change-Id: Id4d5fdbaff2fe6dc49c38f01e73f900f84d3696b