Don't use MNT_DETACH when umounting APEXes

MNT_DETACH doesn't really guarantee that umount will happen in case
mount point is currently busy. It wasn't the problem before, but with
introduction of userspace reboot we need to make sure that APEX is
unmounted, or fail if for some reason mount point is still busy

Bug: 135984674
Test: atest apexservice_test
Test: atest CtsStagedInstallHostTestCases
Change-Id: I9d3fb91ee613679037601181bda874e519f81adf
2 files changed