Update java.io.File to 8u60

- File fields path/prefixLength are final (and #readObject
is implemented using Unsafe).
- Temp file name generation uses longs. Restored TempDirectory
(removed in 3a4b7ebcde0ba6b7dce5c6118fa6a3da77b3d21d) inner
class, to minimize the diff from upstream.
- Javadoc and formating updates

Test: vogar run on all io tests I could find.
Bug: 31237296
Change-Id: Ia0ce0ba3db9cbdb4a5a411325ee1ce0d9c4a5d0a
4 files changed