[DO NOT MERGE] Notify vold when to start fstrim.

Since using the discard option for mounting an Ext4 file system
could lead to unpredictable slowdowns of I/O we will explicitly
control when to perform fstrim, i.e. discard unused blocks of the
mounted volume. We are using the idle maintenance intents sent by
the system which specify a window that one can use to perform
expensive maintenance tasks while the user does not use the device.

This change is adding a broadcast receiver for the idle maintenance
start intent to the mount services which then notifies vold when to
start fstrim.

Since fstrim will be run on a daily basis we do not expect fstrim
to be too long, so it is not interruptible. We will implement
interruption only in case we see issues.

bug:8056794

Change-Id: I1438479d2956b61b82d3a130854376f7a144aaf3