init: pre-create loop devices

apexd-bootstrap now needs more than 16(CONFIG_BLK_DEV_LOOP_MIN_COUNT)
loop devices. apexd-bootstrap spends so much time on creating loop
devices and waiting for them visible under /dev. This is also blocked by
ueventd's cold boot.

In this change, init pre-creates loop devices so that apexd-bootstrap
doesn't have to be blocked by ueventd. On a cuttlefish, apexd-bootstrap
finishes in ~250ms (vs. 350ms).

Bug: 413174275
Flag: com.android.apex.flags.mount_before_data
Test: check boottime
Change-Id: I258d7a797e9724e450765ff0a6924ca5694a6316