Directly create ramdisk dirs in ramdisk image rule

These directories have been created with post install cmds of
init_first_stage. To migrate init_first_stage to Soong, the directory
rules are now written directly in ramdisk image rule.

Bug: 187196593
Test: "m installclean; m" and see ramdisk output
Change-Id: Ic76c325ce102347f20b282572e3edbb5b4359aaf