Add apexd --unmount-all

Will be used by the init during userspace reboot (see another CL in this
topic)

Bug: 135984674
Test: adb install system/apex/shim/com.android.apex.cts.shim.v1.apex
Test: adb reboot userspace
Test: verified install succeeded
Change-Id: I46fe24a500b11ad1323c84be4eb6c67494b0dceb
3 files changed