Add build rule for API->source mapping.

This is used to get source positions of all class members in the boot
class path, to automate the addition of annotations. See
go/hidden-api-annotations for more context.

Mappings printed in:
out/soong/.intermediates/frameworks/base/hiddenapi-mappings
    /android_common/dex-mapping.txt

Test: make hiddenapi-mappings
Bug: 110868826
Change-Id: Ie67a8be6a5a71447a25d081bd466b8b5e01b0725