sensors: Handle fod press status without coordinates

Also fix the error handling of sscanf which returns the
number of matched variables on partial success.

Change-Id: I785c0e3f73e89f79addcf18e1b5111e93e25e430

sensors/Sensor.cpp

@@ -42,7 +42,13 @@ static bool readFpState(int fd, int& screenX, int& screenY) {
     }
 
     rc = sscanf(buffer, "%d,%d,%d", &screenX, &screenY, &state);
-    if (rc < 0) {
+    if (rc == 1) {
+        // If only the first variable can be matched assume
+        // that the node only reports the state
+        state = screenX;
+        screenX = 0;
+        screenY = 0;
+    } else if (rc < 3) {
         ALOGE("failed to parse fp state: %d", rc);
         return false;
     }