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

This reverts commit 62359082b83b870195f7899bd33d3d86d1830988.

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

Cherry-picking https://android-review.googlesource.com/c/platform/packages/apps/Email/+/992814

Test: manual - Ran the following tests on Pixel phone. Tested the email UI for notifications.

$ make -j 40
$ make EmailTests -j
$ adb install -r out/target/product/marlin/system/product/app/Email/Email.apk
$ adb install -r out/target/product/marlin/testcases/EmailTests/arm64/EmailTests.apk
$ adb shell am instrument -w com.android.email.tests
  Time: 2.986
  OK (157 tests)

Change-Id: I66beb67a7bb74bd272c65777adbcd1f8d0d12835
Merged-In: I66beb67a7bb74bd272c65777adbcd1f8d0d12835
5 files changed