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

This reverts commit 18f04be2a68d433da1a9f9f38dfe792fb43be498.

Reason for revert: Email app needs to target API level 28+ 
Bug: 138276176
Bug: 119499886

Change-Id: Id46c0c0379bd1bd35023cbd9325d42d090187725
5 files changed