ART: Add Monitor::FetchState

Add a function returning descriptive state for a monitor. The
function will return the thread state, and possible the object
the thread is blocked/waiting/sleeping on, as well as the lock
owner's tid.

Rewrite DescribeWait to use this.

In preparation for new uses.

Bug: 70538431
Test: m
Change-Id: I79d0104ad04dec633522d4127542184a0cbdf95c