AOSP/Gallery2 - Revert the sdkVersion to 28. On blueline, AOSP Gallery app...

AOSP/Gallery2 - Revert the sdkVersion to 28. On blueline, AOSP Gallery app wasn't able to read photo files.

BUG: 145495831
BUG: 133177396
BUG: 143968106

Test: manual - Ran the following on Bonito and Blueline phones. Tested the gallery2 UI manually.

$ lunch aosp_blueline-userdebug
$  . build/envsetup.sh
$ make -j 40
$ adb install -r -d -g out/target/product/blueline/product/app/Gallery2/Gallery2.apk

$ lunch aosp_bonito-userdebug
$  . build/envsetup.sh
$ make -j 40
$ adb install -r -d -g out/target/product/bonito/product/app/Gallery2/Gallery2.apk

Change-Id: I47a71a933a51d541a39c9fba7087510afd15c7d7