Follow "atomic" to "group" refactoring.

Remove noisy logging about UIDs that are relying on default cache
quota of 64MiB.

Move away from yucky old statfs() and use statvfs() instead.

Test: /data/nativetest/installd_cache_test/installd_cache_test
Bug: 35812899, 35684969, 36482620
Change-Id: I3d68da97eac2ebcda489bdf9d27061cac5b3f7cc