Convert ramdisk_node_list to Android.bp

`ramdisk_node_list` is a hardcoded dependency in
`build/make/core/Makefile`, but it only serves as an input for
generating `ramdisk.img` and isn't installed on the device.

Therefore, we can directly use a `filegroup` and set
`export_to_make_var` for use in build/make/core/Makefile.

In addition, since there are no `Android.mk` files in the
subdirectories of `system/core/rootdir/`,
`include (callall−makefiles−under,(LOCAL_PATH))` can also be
deleted.

Bug: 359423289
Test: m -j $(PRODUCT_OUT)/ramdisk.img
Change-Id: Ia2bf636bfd7917bf7044d9726b4b52932bb161c8