Add SkQPAssetManager::iterateDir to find files in a directory.

We can't use SkOSFile::Iter in the Java version of SkQP--it doesn't find
any files. We need to load our assets via the AAssetManager.

Change-Id: Id9c58fb83da810884d2fa4272fc58101e436aee7
Bug: skia:13042
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/521841
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed