Make Job.failureCount volatile

This allows Job.hasFailures() to be called from the progress reporting
thread without the need for extra synchronization.

Also removed some synchronized blocks from UnpackJob, which are not
necessary anymore.

Bug: 416381187
Flag: EXEMPT fixing synchronization issues in Job class
Test: atest DocumentsUIGoogleTests:com.android.documentsui.services

Change-Id: I0ae1f1787d8e4f2946a5ded0f902ff9a29546c98