Write custom recovery UI vars as recovery build prop.

We used to consume target-specific recovery UI variables at build time,
by passing them as macros, which prevents from building recovery with
Soong. This CL writes these vars as build properties, which can be
queried by recovery at runtime.

With this CL (and the change to recovery), it also allows customizing
these properties at boot time, e.g. by init based on the device SKU.

Bug: 110380063
Test: `m bootimage` with aosp_taimen-userdebug. Check that recovery build
      property file ($OUT/recovery/root/prop.default) has
      `ro.recovery.ui.margin_height=105`.
Change-Id: I53409593779e604b8d8c727d522d9eb76e14508a