Require people who check in new prebuilts to decide what keys they
should be signed with, rather than having a fire drill every time we
go to sign a build and discover new prebuilts.
If a prebuilt specifies a LOCAL_CERTIFICATE, stick it in the
PACKAGES.* collection so that the predexopt process can work on the
file (which only works if the new .apk can be signed).
Change things so that when $(LOCAL_PREBUILT_STRIP_COMMENTS) is
nonempty, we copy the source file using sed to strip out all the "#
line"-style comments and blank lines, saving considerable system image
space in the case of some wifi configurations.
Bug: 2036961
