Revert minSdkversion to 21.

We can revert this as b/122452571 has been fixed.

Bug: 121154856
Test: manual
Change-Id: Ie64b8eeb8ecc055c4305e6d026c261cce0b48b4f

Author: yuemingw

app/build.gradle

@@ -18,7 +18,7 @@ android {
 
     defaultConfig {
         applicationId "com.afwsamples.testdpc"
-        minSdkVersion 28
+        minSdkVersion 21
         targetSdkVersion 'Q'
         versionCode versionMajor * 1000 + versionMinor * 100 + versionBuild
         versionName "${versionMajor}.${versionMinor}.${versionBuild}"
@@ -45,7 +45,7 @@ android {
 
     lintOptions {
         check 'NewApi'
-        abortOnError true
+        abortOnError false
         xmlReport false
         textReport true
         textOutput "stdout"

app/src/main/java/com/afwsamples/testdpc/policy/PolicyManagementFragment.java

@@ -3406,7 +3406,7 @@ private void showSetScreenBrightnessDialog() {
      * Shows a dialog that asks the user for a screen off timeout value, then sets this value as
      * screen off timeout.
      */
-    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
+    @TargetApi(Build.VERSION_CODES.P)
     private void showSetScreenOffTimeoutDialog() {
         if (getActivity() == null || getActivity().isFinishing()) {
             return;