Message: Initialize heapIndex to -1

MessageStack.removeMessage expects a zero or positive value here to
indicate that the message was inserted into a heap.

Without this we were hitting a race where a message could be inserted
into the stack after MessageQueue.nextMessage() called
MessageHeap.heapSweep() but before the call to
MessageHeap.drainFreelist() was made. This would fool
MessageHeap.drainFreelist() into thinking the message needed to
be removed from a heap when in fact it was never inserted into one.

Bug: 421623328
Flag: EXEMPT - bugfix
Test: atest CtsOsTestCasesRavenwood:MessageQueueTest#testStressQuit
Change-Id: I037111ab1e021d8d734b13756aa6e6a0de619232