Generate recovery.img for unittest during build time

The unit tests for imgpatch is comparing the compressed bytes. As
a result, these unit tests will fail with libz change.

Since the recovery image is just a gzipped ramdisk with some wrappings,
we can generate with minigzip it during the build time. This matches
the usage in the real world, where we generate the patch with the host
side libz; and apply the patch with the library on the device.

Bug: 149443852
Test: tests pass on Pixel3
Change-Id: I7885765a161c6bf765671bc55a72cfcaa04b4138