init: "init subcontext" in default mount namespace

"init subcontext" is responsible to handle a few commands in vendor
context. To support Vendor APEX, "init subcontext" is now started in
"default" mount namespace so that those commands can access /apex/*.

For example, insmod should see the contents of /apex to install kernel
modules in apexes.

Bug: 162701747
Test: adb shell ps -ef | grep init
      => check pid of <init subcontext>
      adb shell readlink /proc/<pid: init>/ns/mnt
      adb shell readlink /proc/<pid: init subcontext>/ns/mnt
      => check if these are equal
Change-Id: Icd5e862e52d78635e83824b267b6d32d8f3e9eea