SampleApp: remove SkWindow::onPDFSaved, it does not work

SampleApp: remove SkWindow::onPDFSaved, it does not work

Remove SkWindow::onPDFSaved, it does not work and it is not used anymore.

Also, it is very SampleApp -specific feature implemented in SkWindow class.
SkWindow class is probably intended to be more app-agnosting than
what this method implies.

Presumably the idea of the callback was to inform the user of
SampleApp that the PDF was saved to a certain location.

For Android SampleApp, show a hardcoded toast that the PDF was saved to
/sdcard.

BUG=skia:4733
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Review URL: https://codereview.chromium.org/1547923002