Set `TARGET_RECOVERY_UI_LIB` as a Soong config string list

`TARGET_RECOVERY_UI_LIB` should be set as a string list using
`soong_config_set_string_list`, not as a string, because it usually
specifies multiple libraries.

Bug: 375341427
Test: m -j librecovery_ui_ext
Change-Id: I90f6d6e4d591f401ccac7a23100a5b2e1a5db639