Install Mozc into `%ProgramFiles%\Mozc` rather than `%ProgramFiles(x86)%\Mozc`

## Description
Historically we have been installing Mozc into `%ProgramFiles(x86)%\Mozc` (e.g. `C:\Program Files (x86)\Mozc`) rather than `%ProgramFiles%\Mozc` even on 64-bint environment as that is where Google Japanese Input has been instaleld since 2009.

This is going to be much more confusing when we start supporting Windows Arm64 environment, where we may see the following 3 "Program Files" directories.

* `C:\Program Files`: 64-bit binaries for both Arm64 and x86_64
* `C:\Program Files (Arm)`: arm 32-bit binaries
* `C:\Program Files (x86)`: x86 32-bit binaries

Let's see if we can migrate from `%ProgramFiles(x86)%\Mozc` to `%ProgramFiles%\Mozc` without disturbing existing (Google Japanese Input) users, as one of preparations to support Windows Arm64 environments.

## Steps to reproduce
Steps to reproduce the behavior:
1. Install some older version of Mozc, which is to be installed into `%ProgramFiles(x86)%\Mozc`.
2. Launch some app and select Mozc.
3. Build a new version of Mozc, which is to be installed into `%ProgramFiles%\Mozc`.
4. `msiexec.exe /i Mozc64.msi /qn REBOOT=ReallySuppress`

## Screenshots
Mozc continues working on the app launched at the step 2.

## Version or commit-id
8c8a4461febab02477b688e687765b67b6fd8927

## Environment
- OS: Windows 11 23H2

## Additional context
* https://github.com/google/mozc/issues/732
* https://github.com/google/mozc/discussions/893