[TextInputLayout] Add a dropdown to the left of input instead of icon

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**Is your feature request related to a problem? Please describe.**
In my UI design I need to add a dropdown to the left side of a TextInputLayout. Currently there is only support to add a drawable icon.

**Describe the solution you'd like**
It would be great to add another child to a `TextInputLayout`, name it `LeftDropdown`, for example.

```xml

```

The dropdown could receives its options like the Spinner view already does, through an Adapter.

**Describe alternatives you've considered**
My workaround was to add a transparent left drawable icon and the Spinner component on top of the TextInputLayout, like this:

```xml

```

**Additional context**
n/a

Start by reviewing the existing TextInputLayout API and the Spinner adapter usage described in the issue. Done would mean a supported dropdown can appear on the input's left side, accept adapter-provided options, and work through the requested XML structure.

Written by the indexing model from the issue text.