Control hidden API toast with a system property

CL Id0a4200f912ac3303026cb26b6d8974c47332828 sets a system property
"ro.art.hiddenapi.warning" for non-release, non-user builds. This
patch reads that flag and unless the flag is set, will only ever show
the warning message if the app is debuggable.

Test: manual
Bug: 64382372
Change-Id: I9b552792779589a7a91818a82d5c86141fc0a30b