Lazy unmount postinstall if it fails to unmount.

If postinstall forks a new child that's not killed when killing the main
postinstall process or if postinstall makes any other running process to
hold a file descriptor open in the mounted filesystem, the filesystem is
busy an we can't unmount /postinstall. Since the postinstall mountpoint is
fix in Android, we need to force a lazy unmount in order for the process
to succeed in a future run.

This case can only occur when canceling an update during postinstall (for
example if the update was retried from the server) and would otherwise
clear itself after a few unmount retries if the process using the fd
stops using it.

Bug: 31021934
Test: Added native tests to excersice this case.

(cherry picked from commit 5ba93ad4)

Change-Id: I3464377b43e2ee79cd7318dbc9d3a62706c6ea6c