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
(cherry picked from commit e9671affb68436d70bbf81109baac5ce69fc836c)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:014bfed1befc6e3a659b178f376d2ed5f8174d97
Merged-In: I523c01bddf4f8b084c1f102dab2d8aca26222ec3
Change-Id: I523c01bddf4f8b084c1f102dab2d8aca26222ec3