mw.loader: Use requestIdleCallback to update module store

mw.loader: Use requestIdleCallback to update module store

.. instead of setTimeout.

Also adapt the code accordingly since it is no longer needed to
cancel and re-schedule an update every time update() is called.
This is already handled by requestIdleCallback.

We can let the already-scheduled callback survive. The data
that will be saved is serialised during the flush anyway,
not when scheduling.

Change-Id: Iea48919b4baba7647040b6ed9ff4b6d0d387b962