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