Add detailed guide for file_list_diff error

# Resolving System Image File List Differences

The Android build system uses the `file_list_diff` tool to ensure consistency

between the lists of installed files in system images defined by Kati and Soong.

This check is crucial when transitioning to Soong-defined system images. If the

tool detects any discrepancies, the build will fail.

This document helps you understand and resolve the reported errors. There are

two main types of errors: files present only in the Kati-defined image

(`Kati only`) and files present only in the Soong-defined image (`Soong only`).

## Understanding and Fixing Errors

### Kati only installed files

This error indicates that certain system modules are included via

`PRODUCT_PACKAGES` in your device's Makefiles (`.mk` files) but are not

explicitly defined within the `android_system_image` module or its default

dependencies in `Android.bp`.

**To resolve this:**

* **Default System Modules:** If the module is defined in a common system

Makefile (like `base_system.mk`, `generic_system.mk`, etc.), ensure it's listed

in the `system_image_defaults` module within

`build/make/target/product/generic/Android.bp`.

* **Device-Specific Modules:** For modules specific to your device, add them to

the relevant `android_system_image` module defined in

`PRODUCT_SOONG_DEFINED_SYSTEM_IMAGE` for your target.

### Soong only installed files

This error means that certain system modules are present in the Soong-defined

system image (specified by `PRODUCT_SOONG_DEFINED_SYSTEM_IMAGE`) but are not

included in the `PRODUCT_PACKAGES` list for your target.

**To resolve this:**

* **Remove Incorrect Modules:** If these modules shouldn't be part of the system

image, remove them from the `android_system_image` module definition.

* **Add Missing Modules to Makefiles:** If these modules are indeed required,

add them to the appropriate `.mk` files, following the guidance in the "Kati

only installed files" section to ensure they are correctly included in both the

Kati and Soong definitions.

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    if unique_in_kati:

        print('')

        print(f'{COLOR_ERROR}Missing required modules in {system_module_name} module.{COLOR_NORMAL}')

        print(f'To resolve this issue, please add the modules to the Android.bp file for the {system_module_name} to install the following KATI only installed files.')

        print(f'You can find the correct Android.bp file using the command "gomod {system_module_name}".')

        print(f'{COLOR_ERROR}Missing required modules in the "{system_module_name}" module.{COLOR_NORMAL}')

        print(f'To resolve this issue, please add the modules to the Android.bp file for the "{system_module_name}" to install the following KATI only installed files.')

        print(f'You can find the Android.bp file using the command "gomod {system_module_name}".')

        print('See build/make/tools/filelistdiff/README.md for more details.')

        print(f'{COLOR_WARNING}KATI only installed file(s):{COLOR_NORMAL}')

        for item in sorted(unique_in_kati):

            print('  '+item)

    if unique_in_soong:

        print('')

        print(f'{COLOR_ERROR}Missing packages in base_system.mk.{COLOR_NORMAL}')

        print('Please add packages into build/make/target/product/base_system.mk or build/make/tools/filelistdiff/allowlist to install or skip the following Soong only installed files.')

        print(f'{COLOR_ERROR}Missing packages in PRODUCT_PACKAGES.{COLOR_NORMAL}')

        print(f'Please add packages into .mk files or remove them from the "{system_module_name}" module in Android.bp file.')

        print(f'You can find the Android.bp file using the command "gomod {system_module_name}".')

        print('See build/make/tools/filelistdiff/README.md for more details.')

        print(f'{COLOR_WARNING}Soong only installed file(s):{COLOR_NORMAL}')

        for item in sorted(unique_in_soong):

            print('  '+item)