screenshot: allow customized name for ImageExporter's expoerted file

- Add an interface to allow user customize the name of file output by
  ImageExporter.
  - When there is existing file with the same name and format as the
    newly exported one, the old one will be overrwite by the new.
    Instead of keeping both and add number at the end of the file name.
    (Like, screenshot.png and screenshot (2).png)
- Edited the comment to differentiate existing interface from the new one.

Flag: None
Test: atest ImageExporterTest
Bug: 326477108
Change-Id: I3a5d779f6ec24169c2905f8b78d23aa91c01f7ad