A sysprop apexd.status is set by apexd, to that other components (i.e.
init) can determine whether APEXs are all successfully mounted or no
(i.e., being mounted).

The sysprop is only writable by apexd.

Bug: 117403679
Test: adb shell getprop apexd.status returns 'ready'.
Change-Id: I81bcb96e6c5cb9d899f29ffa84f91eab3820be25