ANDROID: uid_sys_stats: fix negative write bytes.

A task can cancel writes made by other tasks. In rare cases,
cancelled_write_bytes is larger than write_bytes if the task
itself didn't make any write. This doesn't affect total size
but may cause confusion when looking at IO usage on individual
tasks.

Bug: 35851986
Change-Id: If6cb549aeef9e248e18d804293401bb2b91918ca
Signed-off-by: Jin Qian <jinqian@google.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>