Docs: clarify onShowFileChooser javadoc

No change to logic. This only updates and clarifies the
onShowFileChooser documentation. This provides actual instructions for
how to show a file chooser API and explains the default.

Bug: None
Test: None
Flag: DOCS_ONLY
Change-Id: Ia40a08002ab6621ef2adc7c10e6027cde87dcf01