Workaround bogus Thread Safety Analysis warning

Building art/runtime/gc/heap.cc with thread safety analysis produces
warnings that mutator_lock_->AssertNotHeld() conflicts with
ScopedObjectAccess for holding shared and exclusive lock within the same
scope. AssertNotHeld() is a capability assertion and does not actually
hold the lock.

Thread safety analysis was broken thus the warning not appearing before
Clang r316199 update. Temporarily workaround the issue while we fix the
bug in upstream.

Bug: 71769596
Test: m checkbuild
Change-Id: I48234db966332cf24e40774cd62589359aab6d05