ObjC EncodedImage: Use fixed width integer types

We currently use long for some variables, which causes warnings when
converting from int64_t. We should use fixed width integer types
instead.

BUG=b/65491700

Review-Url: https://codereview.webrtc.org/3009293002
Cr-Original-Commit-Position: refs/heads/master@{#19791}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 43467b09c88533d7ecf2659bb7cd66ed423c7f7c