AAPT2: Add -A (assets) support

Looks like the build system doesn't support assets/ for
resources, so we will re-introduce them in aapt2, even though
we're just copying them around and they would be better
suited for inclusion in the APK when classes.dex gets inserted.

Bug: 35461578
Test: CTS test android.content.res.cts.AssetManager#testAssetOperations should pass
Change-Id: I18361d7367d476806bcf7154ee76df3f0e83b565
diff --git a/tools/aapt2/util/Files.h b/tools/aapt2/util/Files.h
index 95c492f..b3b1e48 100644
--- a/tools/aapt2/util/Files.h
+++ b/tools/aapt2/util/Files.h
@@ -132,6 +132,11 @@
   std::vector<std::string> pattern_tokens_;
 };
 
+// Returns a list of files relative to the directory identified by `path`.
+// An optional FileFilter filters out any files that don't pass.
+Maybe<std::vector<std::string>> FindFiles(const android::StringPiece& path, IDiagnostics* diag,
+                                          const FileFilter* filter = nullptr);
+
 }  // namespace file
 }  // namespace aapt