experimental/minimal_mtl_app: more cleanup

Try to keep a consistent style for objective-C++ code.

Also, share code between both apps.

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Mac-Clang-arm64-Debug-iOS_Metal,Build-Mac-Clang-arm64-Release-iOS_Metal

Change-Id: I4b749780c81319bd048f4295c21dfa91910e262d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/241042
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
2 files changed