apexd is started much earlier to create loopback devices

This change fixes the problem that apexd is delaying the entire boot
sequence while waiting for the loopback devices to be created. The delay
was as big as 50 ms per a loopback device.

With this change, apexd is started much earlier: from "on post-fs-data"
to "on init". When it is first started, it scans /system/apex to
determine the number of APEXes and creates that number of loopback
devices priori. Since then it enters into the binder loop.

When the data partition is mounted, init lets apexd to initiate the
apexd boot sequence where APEXes in /data is scanned, verified, and
activated. Since the creation of the loopback devices were requested far
before, it is very likely that dev nodes for the devices are ready at
this moment (even if not, this isn't a lose).

Bug: 123404717
Bug: 123772265
Test: compare boot times.
init_zygote_START_TIME_avg is improved from 2831ms to 2622ms on blueline
Change-Id: I12450cee44aa4d17a11def62261c2f82d3f2c718