Make init builtin command installkey respect property updates

init used to block on installkey such that it was unable to
process property events. This lead to a deadlock by which
the Keymaster HAL would wait indefinitely for the
hwservicemanager.ready=true.

This fixes the issue by implementing the builtin in terms of
do_exec, which allows init to stay responsive to properties
while waiting for the child to terminate.

Bug: 36278706
Test: Add a 3s delay into hwservicemanager before it sets the property
      hwservicemanager.ready and the device still boots.

(cherry picked from commit 9cc51728)

Change-Id: Ia241059d95c610ab01a314969ba23c003333bbba