Fix potential deadlock with SensorObserver.

Sensor Observer grabs it's own lock (mSOLock) when voting for a display
mode, which then grabs the global DMD lock (mLock).
When printing out the dump() method, DMD holds mLock, then calls into
SensorObserver which grabs the mSOLock.  This can cause a deadlock.

There's no reason to hold mLock when calling SensorObserver's dump()
method, so fix that.

Test: adb shell dumpsys display
Bug: 210086128
Change-Id: Idf685f94b818f04f30bee5b85a503f64d448291a