Add contents property to boot_image (and prebuilt_boot_image)

Allows boot_image modules to be created for any module that contributes
to the boot class path, e.g. core-i18n from the com.android.i18n.

A boot_image module with a contents property cannot specify an
image_name, and vice versa. Only those boot_image modules with an
image_name create .art, .oat and .vdex files, either in their
associated APEX or as part of the framework "boot" image.

Bug: 177892522
Test: m nothing
Change-Id: Idfc2bcf00dd6d3ed36ac4df46fcf18e8aa7e2c92