.DS_Store inside *AppIcons.xcassets causes validation error

If a user opens a `*AppIcons.xcassets` in the finder ( or creates a `.DS_Store` in some other way ), it will cause a validation error.

```
ERROR: /Users/jerrymarino/Projects/ios-review/Pinterest/iOS/App/BUILD:81:1: in ios_application rule //Pinterest/iOS/App:Pinterest: Traceback (most recent call last):
File "/Users/jerrymarino/Projects/ios-review/Pinterest/iOS/App/BUILD", line 81
ios_application(name = 'Pinterest')
File "/private/var/tmp/_bazel_jerrymarino/dab264afbccc7ab80d8809aaba91b923/external/build_bazel_rules_apple/apple/bundling/ios_rules.bzl", line 78, in _ios_application_impl
bundling_support.ensure_single_xcassets_type("app_icons", app_icons, "appiconset")
File "/private/var/tmp/_bazel_jerrymarino/dab264afbccc7ab80d8809aaba91b923/external/build_bazel_rules_apple/apple/bundling/bundling_support.bzl", line 322, in bundling_support.ensure_single_xcassets_type
_ensure_path_format(attr, files, [["xcassets", extension...], ...)
File "/private/var/tmp/_bazel_jerrymarino/dab264afbccc7ab80d8809aaba91b923/external/build_bazel_rules_apple/apple/bundling/bundling_support.bzl", line 407, in _ensure_path_format
fail(("%s, but found the following: %...)), ...)
attribute app_icons: Expected the xcassets directory to only contain files are in sub-directories with the extension appiconset, but found the following: [
Pinterest/iOS/App/Resources/ProdAppIcons.xcassets/.DS_Store
]
```

If we don't think this is a good idea to add a special case for `DS_Store` in the validation logic, I can add it to my rule:

```
ios_application(
name = "Pinterest",
app_icons = glob(["Resources/ProdAppIcons.xcassets/**"], exclude=["*.DS_Store"]),
```
but it seems suboptimal for everyone to have to add this.

Start at bundling_support.ensure_single_xcassets_type and _ensure_path_format in the referenced apple bundling files, then trace how app_icons paths are validated. Reproduce the .DS_Store case and verify that an xcassets directory containing it no longer causes validation to fail.

Written by the indexing model from the issue text.