docs: Update ProgressBar JavaDoc comments

- link to material design guidelines for a visual example of
determinate/indeterminate patterns
- reword introduction for clarity
- remove confusing code example, in favor of a concise explanation of
determinate/indeterminate uses
- adds a note about not using progress bar in a dialog

Test: docs change only. Tested with a docs build
Change-Id: I872f7a647eafa6d0f1652549ad2d9d2db4fc81e3