ART: Fix lock order issue in allocation tracking

(Un)instrumenting requires the instrumenting lock, which must be
acquired before the mutator lock. As the plugin can be called both
with and without the lock held, be careful. For simplicity, acquire
the lock (potentially) and immediately suspend.

Bug: 31684277
Test: m test-art-host
Change-Id: Ia6cee2cbe90f13f5543bdfea815d469b28a0f8f4