Add debug ramdisk variant.

A module will be installed to /debug_ramdisk if debug_ramdisk is set to
true.

This is a reland of f84e9c05, with a fix
that removes /first_stage_ramdisk.

Bug: 184004542
Test: soong test
Change-Id: I739de63cfec6b0fec5a90f7c4741fc4d884d209c