fs_mgr: overlayfs: test: noatime on rw mounts only

Regression from commit 808763e8
("fs_mgr: overlayfs: noatime").  Test fails on some targets
that do not follow our advise to mount noatime for all partitions.
blueline mounts the system partitions relatime.  In the grand
scheme this is not a problem because we never remount these
partitions read-write because of the overlayfs integration.

Filter out read-only mounts from those enumerated for noatime,
those failures do not concern us in the context of this test.

This test expected to fail if CtsOsTestCases EnvironmentTest fails.

Test: adb-remount-test.sh
Bug: 140253722
Bug: 140014856
Change-Id: I778e7f539ac3f3d6495867e264767f2af25e11c7