[TextInputLayout] PasswordToggle should only trigger TextWatcher methods if text size > 0

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**Is your feature request related to a problem? Please describe.**
I'm trying to validate an input upon text change via TextWatcher but noticed that the password toggle triggers the TextWatcher methods as well, even when the textfield is empty.

**Describe the solution you'd like**
I would like the password toggle to only trigger TextWatcher methods when text size > 0, or only when there's a text change in the textfield.

**Describe alternatives you've considered**
Here's a bit of a hacky way that works for me for now: https://stackoverflow.com/questions/47096989/textinputlayout-passwordtoggle-handle-click-event/47097367#47097367

**Additional context**

##### Sample:

##### Discussion:
https://twitter.com/waseefakhtar/status/1333858485445009413

Let me know if this is something you'd like to add and I can happily open a PR.

The issue names TextInputLayout, PasswordToggle, and TextWatcher but provides no file or test location. Reproduce the empty-field toggle behavior, trace how the toggle invokes TextWatcher callbacks, and add coverage showing callbacks occur only when the text actually changes or is non-empty.

Written by the indexing model from the issue text.