Add TARGET_RECOVERY_UI_LIB to soong_config_variable

In Android's BoardConfig, `TARGET_RECOVERY_UI_LIB` is a variable used
to specify the Recovery UI library used by the device in Recovery mode.

This `TARGET_RECOVERY_UI_LIB` is used by `librecovery_ui_ext`.
To convert `librecovery_ui_ext` to Android.bp, this variable needs to
be added to soong_config_variable for Soong to use.

Bug: 339143524
Test: m -j librecovery_ui_ext
Change-Id: I2cf01034f02e783e6bff8ce6135b9f7d1a3c62c9