Build first-stage init in Soong

The first-stage init has been built in Make due to some requirements
(like placing it directly under the root directory rather than bin/, and
creating mountpoints like /proc, etc.) that are not supported in Soong.

However, Ie06dc5a93635ea8b1e18be517ed8615b6c82fee6 will make it possible
to satisfy the requirements in Soong. The build of the boot image is
done in Soong and we can create mount points using the `dirs` property
and create a symlink /init that points to /bin/init_vendor using the
`symlinks` property.

To complete the picture of build everying in Soong, this change adds a
Soong-version of the first-stage init.

Note that the Soong-based boot image creation is currently only for the
microdroid usecase. Therefore, the Android.mk-based first-stage init
still remains and will be removed later.

Bug: 178562516
Test: m init_first_stage_soong

Change-Id: I278cb60a11d94fb48341fd3592be0652a25bdbfb