android:path attribute is respected for fields in a slice of struct

This change fixes a bug that android:path attribute is ignored when
annotated field is in a slice of struct.

blueprint now traverses fields whose type is slice of struct and
provides the index list in the case. Soong is modified so that it checks
whether the field being referenced is a slice of struct or not. If that
is the case, it gathers field values from each of the elements. If not,
it follows the original path.

Bug: 181018147
Test: m nothing
Change-Id: I220efb6feaa525a00939654459b2998e98e7ad56