Convert librecovery_ui_ext to Android.bp

Note: After converting `librecovery_ui_ext` to `Android.bp`, for the
modules set in `TARGET_RECOVERY_UI_LIB` in the original
`BoardConfig.mk`, if `soong_namespace{}` was originally set, they need
to be changed to their `fully qualified name`.

Bug: 339143524
Test: m -j librecovery_ui_ext

Merged-In: I85787a9ebe95a709c7ac6977218511c113263294
Change-Id: I85787a9ebe95a709c7ac6977218511c113263294