do not read /proc/sys/vm/drop_caches

/proc/sys/vm/drop_caches has recently been made write-only. Update the
expected permissions and do not attempt to read the file as part of the
test.

Older platforms will continue to have the previous behavior so do not
enforce there.

Bug: 147362634
Test: run vts test on cuttlefish
Change-Id: Ia4c8207e2cec8bd0c556456d60d99c3afcd94dc4
3 files changed