Be increasingly aggressive about fstrim if it isn't being run

The current heuristics depend on devices being alive at midnight+ in
order to run periodic background fstrim operations.  This unfortunately
means that people who routinely turn their devices off overnight wind
up with their devices *never* running fstrim, and this causes major
performance and disk-life problems.

We now backstop this very-friendly schedule with an increasingly
aggressive one.  If the device goes a defined time without a background
fstrim, we then force the fstrim at the next reboot.  Once the
device hits the midnight+ idle fstrim request time, then we already
aggressively attempt to fstrim at the first available moment
thereafter, even if it's days/weeks later without a reboot.

'Available' here means charging + device idle.  If the device never
becomes idle then we can't do much without rendering an in-use device
inoperable for some number of minutes -- but we have no evidence of
devices ever failing to run fstrim due to this usage pattern.

A new Settings.Global element (type 'long', called
"fstrim_mandatory_interval") is the source of the backstop time.  If
this element is zero or negative, no mandatory boot-time fstrim will
ever be performed.  If the element is not supplied on a given device,
the default backstop is 3 days.

Adds a new string to display in the upgrading dialog when doing
the fstrim.  Note it is too late for this to be localized, but since
this operation can take a long time it is probably better to have
it show *something* even if not localized, rather than just sit there.

Bug 18486922

Change-Id: I5b265ca0a65570fb8931251aa1ac37b530635a2c