cmd/dist: update android testing TODO, add iOS

cmd/dist: update android testing TODO, add iOS

This CL updates a TODO on a condition excluding a lot of tests on
android, clarifying what needs to be done. Several of the tests should
be turned off, for example anything depending on the Go tool, others
should be enabled. (See #8345, comment 3 for more details.)

Also add iOS, which has the same set of restrictions.

Tested manually on linux/amd64, darwin/amd64, android/arm, darwin/arm.

Updates #8345

Change-Id: I147f0a915426e0e0de9a73f9aea353766156609b
Reviewed-on: https://go-review.googlesource.com/7734
Reviewed-by: Burcu Dogan <jbd@google.com>