docs: Described why ProgressDialog is now deprecated.

Per feedback from DevRel, devs should avoid ProgressDialog because
using a modal dialog to show progress is a bad user experience.
Updating the ProgressDialog javadocs to say this; there's a separate
CL (http://cr/160568896) to make a similar note in the Dialogs API
guide.

Doc is staged to:

http://go/dac-stage/reference/android/app/ProgressDialog.html

Test: make ds-docs
Bug: 37565313
Change-Id: I189732fdda4532f248861e3f3d077f743f6387de