ART: Refactor JIT stress mode a runtime debug flag

Do not use a constexpr default compilation threshold for the JIT.
Instead use a tri-state that distinguishes non-debug, fast-debug
and slow-debug modes.

Bug: 35644369
Test: m test-art-host
Change-Id: I3f8d71126af6b7f6b6aaa7a59cf0a42e5ee3fe01