[Unpackaged] Fix taskbar glomming due to AUMID (#20064)

The bug was caused by an AUMID mismatch between the Taskbar's .lnk file
and Windows Terminal. Since no AUMID was associated with the .exe, the
OS automatically creates one for us. However, #20018 added an AUMID for
unpackaged scenarios, so now there was a mismatch, resulting in a new
taskbar entry being created.

To fix this, we check if a .lnk points to our .exe in the taskbar.
There's 3 cases here:
1. no .lnk of interest exists --> set the AUMID normally
2. the .lnk carries our AUMID --> set the AUMID normally
3. the .lnk doesn't have an AUMID (aka uses the auto-resolved one) -->
- for this launch: don't set the AUMID so that we both use the
auto-resolved one
- on next launch: set the AUMID on the .lnk and process so that they all
agree

## Validation Steps Performed
In unpackaged folder, move WindowsTerminal.exe to the taskbar (creates
.lnk)...
✅ Double-click the .exe --> Same taskbar entry is used
✅ Double-click the .exe _again_ --> second window goes to same taskbar
entry

The first window doesn't have to close for this to work. It just * works
*!

Bug introduced in #20018 
Closes #20053