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

build.gradle

@@ -24,8 +24,8 @@ android {
     useLibrary 'android.test.mock'
 
     defaultConfig {
-        minSdkVersion 18
-        targetSdkVersion 23
+        minSdkVersion 21
+        targetSdkVersion 28
         versionName "1.0"
 
         applicationId 'com.android.inputmethod.latin'

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" />

tests/AndroidManifest.xml

@@ -18,7 +18,7 @@
     package="com.android.inputmethod.latin.tests"
     android:versionCode="28">
 
-    <uses-sdk android:minSdkVersion="14" android:targetSdkVersion="21" />
+    <uses-sdk android:minSdkVersion="21" android:targetSdkVersion="28" />
 
     <uses-permission android:name="android.permission.READ_CONTACTS" />