Store method as ArtMethod* instead of jmethodId in Breakpoint

Previously we needed a ScopedObjectAccess for Dbg::VisitRoots,
this could cause deadlocks in the following scenario:

GC:
Goes to runnable state while holding mutator lock as shared held.
This occurred in Dbg::VisitRoots when calling Breakpoint::Method.

Other thread:
Calls SuspendAll and suspends the GC thread before it can go back
to suspended thread state. This thread then attempts to exclusive
lock mutator lock, but the GC is suspended while holding it in a
shared state.

Bug: 25336094
Change-Id: Idcb8d34c314b1d6951abe533a0cfa586cc07d7d6