image_signing: Support AOSP dev signing keys
image_signing: Support AOSP dev signing keys

Following ag/3536780, aosp_cheets targets have their APKs signed with
keys https://android.googlesource.com/platform/build/+/master/target/product/security/
The image signing script now:
- detects if it's an aosp_cheets or cheets build
- checks the correct signing keys have been used in either case
- logs more information about the Android image

BUG=b:72947583
TEST=manually test sign_framework_apks against AOSP and cheets builds
TEST=aosp_cheets builds prior to ag/3536780 are now failing (expected)
TEST=aosp_cheets builds posterior to ag/3536780 are passing
TEST=cheets builds (before and after) are passing

TEST=check that commenting out ro.product.name in build.prop triggers an
error
TEST=check that an invalid value in ro.product.name triggers an error
BRANCH=None

Change-Id: I72abea5182fbfe76820e3f48831be04f39cb334e
Reviewed-on: https://chromium-review.googlesource.com/904726
Commit-Ready: Nicolas Norvez <3001a977f47a2bc94ec05551872e2ce88b323556@chromium.org>
Tested-by: Nicolas Norvez <3001a977f47a2bc94ec05551872e2ce88b323556@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot