Bump targetSdkVersion to "28" and minSdkVersion to "21"

With this CL, targetSdkVersion and minSdkVersion will be bumped as
follows:

  targetSdkVersion: 23 -> 28
     minSdkVersion: 14 -> 21

As far as we know, there should be no user-visible behavior change on
supported OS versions.

Fix: 119489995
Test: manually verified that LatinIME still is functional
Change-Id: I68f1545778b0c8b44c6850f22f3f5ca25bdb6de0
diff --git a/java/AndroidManifest.xml b/java/AndroidManifest.xml
index 2d0dc4a..79d3373 100644
--- a/java/AndroidManifest.xml
+++ b/java/AndroidManifest.xml
@@ -19,7 +19,7 @@
         package="com.android.inputmethod.latin"
         android:versionCode="28">
 
-    <uses-sdk android:minSdkVersion="14" android:targetSdkVersion="23" />
+    <uses-sdk android:minSdkVersion="21" android:targetSdkVersion="28" />
 
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
     <uses-permission android:name="android.permission.DOWNLOAD_WITHOUT_NOTIFICATION" />