Update docs to reflect new receiver flag enforcement

Android T allows apps to declare a runtime receiver as not exported
by invoking registerReceiver with a new RECEIVER_NOT_EXPORTED flag;
receivers registered with this flag will only receive broadcasts from
the platform and the app itself. However to ensure developers can
properly protect their receivers, all apps targeting U or later
registering a receiver for non-system broadcasts must specify either
the exported or not exported flag when invoking #registerReceiver.
This commit updates the docs to reflect this new requirement.

Bug: 234659204
Test: m -j offline-sdk-docs
Change-Id: I86fe8b66f963064db7420bebb502c2d125327bc0