Quick fix for possible deadlock in LMS

LMS dispatches in-process location events while holding locks, which
can lead to deadlock if combined with another lock. This went from a
possibility to a definitive deadlock with the introduction of the
getCurrentLocation cancel() API. This fix prevents that deadlock by
not holding a lock while canceling. It is still technically possible
for other components to introduce deadlock by using their own locks
within the system process, but as there are very few location clients
inside the system server, and none of them appear to be at risk right
now, a complete fix is delayed until S.

Bug: 149780283
Test: presubmits
Change-Id: I9d202adc65380881969d276c470ef1bf52828e82

location/java/android/location/LocationManager.java

@@ -2562,22 +2562,28 @@ public class LocationManager {
             mRemoteCancellationSignal = remoteCancellationSignal;
         }
 
-        public synchronized void cancel() {
-            mExecutor = null;
-            mConsumer = null;
+        public void cancel() {
+            ICancellationSignal cancellationSignal;
+            synchronized (this) {
+                mExecutor = null;
+                mConsumer = null;
 
-            if (mAlarmManager != null) {
-                mAlarmManager.cancel(this);
-                mAlarmManager = null;
+                if (mAlarmManager != null) {
+                    mAlarmManager.cancel(this);
+                    mAlarmManager = null;
+                }
+
+                // ensure only one cancel event will go through
+                cancellationSignal = mRemoteCancellationSignal;
+                mRemoteCancellationSignal = null;
             }
 
-            if (mRemoteCancellationSignal != null) {
+            if (cancellationSignal != null) {
                 try {
-                    mRemoteCancellationSignal.cancel();
+                    cancellationSignal.cancel();
                 } catch (RemoteException e) {
                     // ignore
                 }
-                mRemoteCancellationSignal = null;
             }
         }