Allow contents and image_name to be specified together

Previously, only one of the contents or image_name properties could be
specified at once which meant that there was no way to create a
prebuilt which lists its fixed contents while at the same time allowing
it to check that that the contents matched what the build configuration
required.

e.g. a prebuilt_bootclasspath_fragment that had image_name: "art",
could not list its contents and also check that those contents matched
the ART_APEX_JARS which the build configuration required.

This change allows contents and image_name to be specified together and
adds a check to make sure that the contents are consistent with the
configuration appropriate to the image_name. The check is only
performed for modules that are active so that a
prebuilt_bootclasspath_fragment which was created without coverage
enabled (the default) would not cause a build failure in a coverage
build unless it was preferred.

Bug: 177892522
Test: m nothing
Change-Id: Ie601f29f707b3f6030fa7d252afa2c4826cc9f8e