fs_mgr: overlayfs clears readonly on scratch devices

Fixes adb-remount-test and flashstation because the scratch device
was set to readonly before.

Test: adb-remount-test

Change-Id: I12551d1ed99fb7cfd04f84fcc2b77d3cb94275b2
Fixes: 122454600
Fixes: 122602260
3 files changed