raise minimum supported API level to 28

This matches the minimum API level required by the Play Store for both
new apps and app updates, so this is not an aggressive requirement:

https://developer.android.com/distribute/best-practices/develop/target-sdk

There were various backwards incompatible improvements to privacy and
securitty from API 23 to API 28. In particular, API 28 introduced the
new SELinux sandbox using a per-app-per-profile security level (MLS) to
provide a unique security domain for each app instance. Previously, MLS
was used to isolate profiles by using per-profile security levels. The
finer-grained security levels prevent apps from directly sharing data
without going through intents, eliminating many common vulnerabilities
and giving the OS better control over how apps can communicate.

core/version_defaults.mk

@@ -321,6 +321,6 @@ ifndef PLATFORM_MIN_SUPPORTED_TARGET_SDK_VERSION
   # Used to set minimum supported target sdk version. Apps targeting sdk
   # version lower than the set value will result in a warning being shown
   # when any activity from the app is started.
-  PLATFORM_MIN_SUPPORTED_TARGET_SDK_VERSION := 23
+  PLATFORM_MIN_SUPPORTED_TARGET_SDK_VERSION := 28
 endif
 .KATI_READONLY := PLATFORM_MIN_SUPPORTED_TARGET_SDK_VERSION