Fix wp and sp comparison bugs

Make clear() actually clear wp m_refs, so that nulls compare equal.

Make equality consistent with < and >, ensuring that a weak pointer
cannot be both equal to and greater than another.

Don't rely on the built-in < and > operators to correctly order
different objects. The standard does not guarantee that, and there is
a risk of compiler relying on that lack of guarantee.

Remove unnecessary comparison overloads, especially those
comparing a wp<> to an sp<>.

Change the remaining wp<> to sp<> comparisons to check for equivalence
of the mRefs pointer instead of the object address, thus eliminating
the dubious equal comparison result for a dead wp<> and an sp<> that
happen to point to the same object address.

Add comparison tests.

Test: Treehugger, boot AOSP, atest RefBase
Bug: 126922090
Change-Id: I15911150e0fc85ace2c4b77d337826e12793c690