StrictMode: fix race condition in onVmPolicyViolation

There's a TOCTOU race condition in onVmPolicyViolation() that can cause
a NullPointerException if multiple threads trigger a violation and a
penalty listener is set. For example:

1. Thread 1 passes the mCallbackExecutor null check and calls execute()
2. T2 passes the same check and then gets preempted
3. Runnable queued by T1 temporarily replaces sVmPolicy with LAX (which
   has a null executor) by calling allowVmViolations()
4. T2 calls execute() on sVmPolicy.mCallbackExecutor, which is now null

Fix it by using the same VmPolicy object throughout onVmPolicyViolation.

Test: atest StrictModeTest
Change-Id: Ifa20253ea936b8d3d8c3719c3278bfaccbdf8275