Fix memory order and race bugs in Refbase.h & RefBase.cpp

Convert to use std::atomic directly.

Consistently use relaxed ordering for increments, release ordering
for decrements, and an added acquire fence when the count goes to
zero.

Fix what looks like another race in attemptIncStrong:
It seems entirely possible that the final adjustment for
INITIAL_STRONG_VALUE would see e.g. INITIAL_STRONG_VALUE + 1,
since we could be running in the middle of another initial
increment.

Attempt to somewhat document what this actually does, and
what's expected from the client. Hide the documentation in
the .cpp file for now.

Remove a confusing redundant test in decWeak. OBJECT_LIFETIME_STRONG
and OBJECT_LIFETIME_WEAK are the only options, in spite of some
of the original comments.

It's conceivable that either of these issues has resulted in
actual crashes, though I would guess the probability is small.
It's hard enough to reason about this code without the bugs.

Bug: 28705989
Change-Id: I4107a56c3fc0fdb7ee17fc8a8f0dd7fb128af9d8
(cherry picked from commit e263e6c6)