motivation: BUILD.gn os too big, also can be reused in modules.
Also: add a BUILD.gn for minimal_ios_mtl_skia_app that uses the
template.
CQ_INCLUDE_TRYBOTS=skia.primary:Build-Mac-Clang-arm64-Debug-iOS_Metal,Build-Mac-Clang-arm64-Release-iOS,Test-iOS-Clang-iPhone7-GPU-PowerVRGT7600-arm64-Debug-All-Metal,Test-iOS-Clang-iPhone6-GPU-PowerVRGX6450-arm64-Release-All
Change-Id: I70ebb9668cce51ea9fa671a3d8c93041fbedcbfa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/241761
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
