ANDROID: Set ALL_KMI_SYMBOLS to CONFIG_UNUSED_KSYMS_WHITELIST.

Kleaf already generates CONFIG_UNUSED_KSYMS_WHITELIST according to
the list of provided KMI symbol lists (kernel_build.kmi_symbol_list
plus kernel_build.additional_kmi_symbol_lists).
CONFIG_UNUSED_KSYMS_WHITELIST is the source of truth, not the wildcard
here, especially if --user_kmi_symbol_list is set, or
kernel_build.kmi_symbol_list or additional_kmi_symbol_lists contains
files in different directories. Use CONFIG_UNUSED_KSYMS_WHITELIST
directly.

For mainline, CONFIG_UNUSED_KSYMS_WHITELIST is not set, so use
an empty file as a placeholder.

Bug: 406094444
Change-Id: Ia62c7a2e40ca607ac6f0d7e91c732044ec559282
Signed-off-by: HONG Yifan <elsk@google.com>