MTP: Fix race conditions in MtpServer JNI code

Make sure previous MtpThread has exited before starting another to avoid
EBUSY opening MTP kernel driver.

BUG: 3317803

Change-Id: I81dcbac42bcf5f680ed1b1469839bc0b0e49d53d
Signed-off-by: Mike Lockwood <lockwood@android.com>
3 files changed