Revert "AOSP/Email - Bump targetSdkVersion to 28."

This reverts commit 62359082b83b870195f7899bd33d3d86d1830988.

Reason for revert: No notification when new email arrived Bug: 132923014

Change-Id: I66beb67a7bb74bd272c65777adbcd1f8d0d12835
5 files changed