Improved error message when multiple annotations.

Old:
Check failed: values.size() == 2u (values.size()=3, 2u=2) Currently only \
signature and one flag are supported

New:
Check failed: flags.size() == 1u (flags.size()=2, 1u=1)
out/target/common/obj/PACKAGING/hiddenapi-flags.csv:36125
Landroid/content/om/OverlayInfo;->isEnabled()Z: Expected one flag, found 2:
greylist,whitelist
See go/hiddenapi-error for help.

Test: m
Change-Id: Iab4bef5d24e416ea43e0302f898fc987f3158489