Notify that ASEC containers are being unmounted during upgrades

When an apk is installed on ordinary unmountable media, a broadcast
is sent when the OS wants to unmount it so that interested parties
can cleanly close any files they have open to read that apk's
resources or similar.  We now send that broadcast when we are
about to unmount the ASEC fs container that holds a forward-locked
apk as well, so that e.g. Home knows to release the resources that
it was using for widget hosting or similar.

Bug 7703848

Change-Id: I71aefdb4086c7b73a128f89c15d192a2b92d09a8