Update Viewer docs for Android and iOS.
Update Viewer docs for Android and iOS.

Change-Id: I4208221f982ce79b6c8af9aaa05644b0f1f94a5e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268680
Reviewed-by: Tyler Denniston <293ae2d31b7e9da32c4fb58bdaaed1f4bef4c6e7@google.com>
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>

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