Track redundant ops when opening the cache.

See https://github.com/JakeWharton/DiskLruCache/issues/28

Change-Id: I4815aed3b40f0d2599ff12a42853a4947feffbca
Use EmptyArray.STRING in Splitter.fastSplit().

This is a minor cleanup after
  https://android-review.googlesource.com/47964

Change-Id: Id8de6c09f3397b044082c275095ff7a7a43185f9
Fix compilation of Enum on JDK 7

Change-Id: I2e58b6a527e4ef1fc7ed3359b8aea88d01a031fb
libcore: Better Enum fix for JDK 7

 * Jerome Pilliet suggests using a cast in the compareTo method instead of
   changing the field visibility. This is a much better fix.

Change-Id: I334105e0922225512a4d0a2d4b6c9c6ec71b9c51