fs_mgr: overlayfs: requires override_creds patch for 4.4+ kernels (taimen)

The original adb-remount-test.sh when certifying kernels allowed a
pass on 4.4 kernels because it added new content, and missed a test
for overriding existing content.  When the test was added to confirm
APEX control of libc.so, it serendipitously added a check for
overriding existing content, which the 4.4 kernel did not allow to
pass.  Update the tests and documentation to reflect this new state
of affairs.

Summary: 4.4 kernel overlayfs driver worked partially without the
patch for override_creds.

Test: adb-remount-test.sh
Bug: 126256072
Change-Id: I979ea59a12bc0b9926826b9b09a7893ab3b9ee7f