Allow librecovery_ui_ext to set InstallInRoot() to true.

`librecovery_ui_ext` is the Recovery UI library used by the device in
Recovery mode. Its default installation directory is
$(TARGET_RECOVERY_ROOT_OUT), not $(TARGET_RECOVERY_ROOT_OUT)/system.
It needs to be able to set InstallInRoot to true to match the original
installation location settings.

Bug: 339143524
Test: m -j librecovery_ui_ext
Change-Id: Ic2f7fd9bacc7d1e320481dd4dc034b5db8e80fa7