init: generate apex-info-list.xml

If a device doesn't support unflattened APEXes, init activates flattened
APEXes instead of apexd.

Because apexd dumps apex-info-list.xml after it activates all APEXes,
init should do the same thing.

Bug: 159585065
Test: lunch aosp_cf_x86_phone_noapex & m & boot
      adb more /apex/apex-info-list.xml
      adb shell ls -lZ /apex/apex-info-list.xml
       -> -rw-rw-rw- 1 root root u:object_r:apex_info_file:s0 ..
Change-Id: I74a4d0ee363761564225097c5949e5ce335ac6dc