TextBoxView causes memory leak for 2 seconds after unloading host control

* .NET Core Version: All versions
* Windows version: 1909
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

**Problem description:**
TextBoxView causes memory leak for 2 seconds after unloading host control.
That's caused by the `DispatcherTimer` `_throttleBackgroundTimer`.
This might not be a problem during usage by a human user, but when i use my custom UI test framework this is a real problem.

**Actual behavior:**
TextBoxView causes memory leak for 2 seconds after unloading host control.

**Expected behavior:**
TextBoxView does not leak memory after unloading host control

**Minimal repro:**
- Create a window containing a `TextBox`
- Create a new instance of the window open/show the window, focus the `TextBox` and close that window multiple times during a 2 second timespan in a loop