ScopedFlock: Refactor it to be a subclass of FdFile.

Makes callers cleaner, since they only have to worry about
dealing with regular File objects that they know will be locked
for the duration of their existence. Prevents issues and other
clunky code relating to acquire

Test: scoped_flock_test, test_art_host
Bug: 36369345

Change-Id: I2c9644e448acde6ddac472d88108c7d9a4e1a892