Now that we have removed mBaseLevel and mMaxLevel from ImageHelper class
in previous two CLs. The initExternal function can be modified to not
pass in base/max level information. This CL also removes immutable from
argument and let caller directly pass in firstLevel to it.

Bug: b/181800403
Change-Id: I595c1faffabc7582af35a9578b2ecdf00c39207f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2800414
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>