Add symlinks support for prebuilt_etc modules

Filenames specified in the "symlinks" property are passed to
LOCAL_MODULE_SYMLINKS, and kati would install these symlinks.

For example:
```
prebuilt_etc {
  name: "foo",
  symlinks: [
    "bar",
    "baz",
  ],
}
```

Installs these files on device:
- system/etc/foo
- system/etc/bar -> foo
- system/etc/baz -> foo

Bug: 157537895
Test: Add some symlinks to a prebuilt_etc module and check artifact
Change-Id: If50844e8a212a966be931117cfdff5bf73aadf25