Re-add default channel on app targetsdk downgrade.

Test: runtest systemui-notification, and manual: adb install the
apk pulled off dogfood device, verify that sending an email
results in a toast pre change and no toast + notification
post change.
Change-Id: Ia3db55959906f38a5dc34cb38a33072c1f2dd64f
Fixes: 38437227