Add ctl.apex_(un)load properties to (un)load apex

These props are supposed to be used by apexd when installing an apex
without reboot. During the installation, apexd will unmount the current
one and mount the new one. Since the path and its contents will be
replaced, anything loaded from the apex should be unloaded before
unmounting. After apexd mounts the apex again, then init should re-read
.rc files from the apex.

This change only addes required properties:
- ctl.apex_load <apex_name>
- ctl.apex_unload <apex_name>
- init.apex.<apex_name> = loaded | unloaded

Bug: 232114573
Bug: 232173613
Test: atest CtsInitTestCases
Change-Id: I4a8942d67912e2f11acc51bec756c4e3c218179a