Updating receiver flag requirements and enforcement.

Since we're no longer planning on enforcing the flag requirement for
exported/not exported in T, we should update the compat change so that
it's disabled, which will allow developers to still test it, change the
error message so it doesn't reference Android T, and remove the
additional system property that enabled stricter enforcement.

Test: atest ContextText
Bug: 225057637

Change-Id: Ibdc079ef7e92954422ddde715b3a4ae358f552e3