Refactor the printing of an sdk snapshot's Android.bp contents

Previously, all of the supported types were handled explicitly using
type switches. This change switches to use reflection instead to allow
it to work with unknown types such as custom structures, support for
which will be added in a follow up change.

Bug: 179354495
Test: m nothing
Change-Id: I197f3f1dbce6d80c0127c99bbe6f836d422a8db9