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
Performance improvement in DiskLruCache.readJournalLine().

Speed up DiskLruCache.readJournalLine() by avoiding memory
allocations from String.split(). For non-CLEAN lines, we
avoid using String.split() altogether and find separators
explicitly, for CLEAN lines we defer to String.split() and
we optimize the underlying Splitter.fastSplit() overload to
avoid unnecessary allocations itself.

On a test journal with 7347 entries (1099 CLEAN) this saves
about 45-50% from ~250ms. On a test journal with 272 entries
(86 CLEAN) this saves about 35-40% from ~10ms. Measured
loadJournal on GN in a tight loop (file contents cached).

If used without the other DiskLruCache.readJournalLine()
improvements, the Splitter.fastSplit() optimization alone
would provide about 60% of the savings. It should also
speed up other code outside the DiskLruCache.

Change-Id: I1d6c6b13d54d8fcba3081f2bb9df701b58f5e143
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