A few changes to the measurement implementation

- Fixed the device crashing bug in JNI code
- Fixed a typo ('lock' => 'clock')
- Added hardware clock discontinuity count in toString()

(cherry picked from commit 3a21b02ee2dc8f2c6c879109757d89a2be5d720a)

Change-Id: I6dbda85abda20da0240ac08b22b81729eae7b6f0