libhwui: Use wc instead of du to monitor file write

du returns the disk space a file occupies, which isn't guaranteed to be
0 even if the file is empty on all file systems. Use "wc -c" instead to
obtain the file size.

Bug: 184878226
Test: The script waits for the file to have content on file system that
uses disk space to store empty files.
Test: Skia capture still works for Pixel.

Change-Id: Ib5f75b6712b476fc521a531fba029f91f4b281b5