bug(windows): installing a transient language based keyboard can still add kbdus.dll at times

**Describe the bug**
On Win10, installing SIL IPA results in US English (kbdus.dll) appearing in the keyboard list; it disappears after restart. This does not appear to happen on Win8.1.

This is probably related to the transient language installation process, but currently uncertain as to when.

**To Reproduce**

1. On a clean install with only khmer_angkor.kmp and English (AU) installed, install sil_ipa.kmp.
2. Observe how English (US) is added to the list of loaded keyboards (keyboard switcher) but not in Setings or Registry.
3. Reboot
4. English (US) will no longer be in the list of loaded keyboards.

**Expected behavior**

English (US) is not added.

**Screenshots**


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**Keyman for Windows/macOS/Linux:**
- OS: Windows 10 x64
- Keyman version: 14.0.264-beta