Clean up AssetManager::scanAndMergeZipLocked

Now that ZipFileRO::startIteration supports prefix/suffix matching,
we can pass dirName to the function for simpler code and slightly
better performance.

(cherry-pick of c796ad0a8be6df4a3b354690dfe5ce1df8136c09)

Change-Id: I0e2ac58de28020c2af8d8e569a97592b09596185
1 file changed