Clean up RandomBitsSupplier.

The reason the array had later items commented out is that the loop
was wrong: it would always choose the _last_ valid device, not the
_first_ as intended.

There were two choices here:
1. fix the loop and have the same list of three devices that OpenSSL uses.
2. remove the loop and just say "you must have /dev/urandom".

This patch goes with option 2.

Change-Id: Ia99be6ba929c7b6c35bed77c19d47de80719973c
Remove dead code.

Change-Id: I389bf967ed9a11de8ecb194bf06f69164ac4da4c
Improve the System.currentTimeMillis/Date/DateFormat/SimpleDateFormat documentation.

15 years later, this stuff is still causing confusion.

Change-Id: Id68d43014b72de2cf620cf1798517106fd68d229