[SearchBar] Support for trailing icon / avatar

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**Is your feature request related to a problem? Please describe.**
Though it is mentioned in the [anatomy](https://github.com/material-components/material-components-android/blob/master/docs/components/Search.md#anatomy-and-key-properties) of a `SearchBar` it seems not to be possible to add a trailing icon / avatar to it.

I tried to add a nested `ImageView` but it seems to be ignored. I also search for public methods, but not even in the class itself I was able to find any hint about `avatar` or `trailing`.

**Describe the solution you'd like**
Nesting an `ImageView` or a public `setter`.

**Describe alternatives you've considered**
Rebuilding the whole `SearchBar` from scratch custom

**Additional context**
It's about item `4`:


We also happily accept [pull requests](https://github.com/material-components/material-components-android/pulls).

Start with the SearchBar implementation and the linked Search.md anatomy section, then inspect how child views and public customization are handled. Determine whether the supported outcome should be a nested ImageView, a public setter, or another API, and consider how the trailing icon or avatar should appear in the component.

Written by the indexing model from the issue text.