docs: Update Button JavaDoc comments

- Simplify first-line description
- Update top-level code sample
- Add caveat about onClick running on main thread
- Simplify button style discussion
- Add distinct See Also section
- Convert future tense to present tense
- Add closing paragraph tags
- Remove references to push-button
- Clean up code introduction sentence
- Use present tense
- Connect reasoning to user experience
- Format threading information as aside
- Change future tense to present tense
- Add transition statement
- Use active language in parameter description
- Use cannot instead of can not
- Use common parlance name for button
- Clean up parameter descriptions

Test: Docs change only. Tested with a doc build run.

Change-Id: Ie0cd25ed4f8372ba5ec58bdf8b7dfce9100ee3fc