Use target sdk for background check on Wear remote intents.

Update the Wear remote intent background check to only be enforced from
Android B target sdk.

We cannot break developers who are currently relying on this behaviour
without proper notice and guidance on how to update their apps.

Flag: EXEMPT bugfix
Bug: 419106561, 319101158
Test: locally build and flash device, tested KakaoTalk open on phone on Wear
Change-Id: I523c01bddf4f8b084c1f102dab2d8aca26222ec3