fix: enforce UTF-8 encoding for file output

[etnperlong]

Description

This PR fixes a UnicodeEncodeError that occurs when processing firmware dumps containing non-ASCII characters in build properties, especially on systems where the default locale encoding is not UTF-8.

Problem

When processing certain Chinese ROMs (e.g., TB375FC_CN), the tool crashes during the proprietary files list generation or build properties dumping phase:

Traceback (most recent call last):

  File "aospdtgen/utils/format_props.py", line 52, in dump_partition_build_prop
    f.write(f"{prop}={build_prop.get_prop(prop)}\n")
UnicodeEncodeError: 'ascii' codec can't encode characters in position 32-33: ordinal not in range(128)

This is because the file is opened using the environment's default encoding (often ASCII), which cannot handle the Unicode characters present in the build properties.

Solution

Ensured that all text-based file writing operations explicitly use UTF-8 encoding:

Verification

The fix has been verified to correctly process ROMs that previously caused the crash.

[SebaUbuntu]

Merged, thanks!