Generate the recovery text images at build time

We used to manually generate the background text images for recovery. That
process requires opening various emulators and takes a long time. Now that
the host Java tool is ready to generate these images, we can embed the
generation step into the build of the recovery image.

Since there's currently no make variable that defines screen width of the
device, we put the commonly used values for pixels here, i.e. 1440 for
xxxhdpi and 1080 for xxhdpi. Devices with other dpi can still use this
feature if they define the screen width in their config files.

Test: build bootimage with screen with set/unset
Change-Id: Ic8389a527e6e7194de58a9c50763e841e886652b