Support custom my_copy_pairs in package-modules.mk

To support extra files in package-modules.mk, allow the user to set
my_copy_pairs to a list of src:dest pairs that will be copied into the
zip file.

Test: build-aosp_arm.ninja is identical before/after
Test: codesearch says that these variables aren't otherwise used
Test: set my_copy_pairs, ensure that they exist in the zip.
Change-Id: Ia80cd136db8ad37a71010baf0552621b281c8bc3