Optimize battery optimize page control flow

- Post the setup logic into background and setup once only when leaving
 this page, this can improve the performance since we won't setup
 every time when switching preference

Bug: 195306545
Bug: 199892006
Test: make SettingsRoboTests
Change-Id: I1c3ee4673cf5cdba0abe39f208dfb1412082c579

tests/robotests/src/com/android/settings/fuelgauge/UnrestrictedPreferenceControllerTest.java

@@ -19,8 +19,6 @@ package com.android.settings.fuelgauge;
 
 import static com.google.common.truth.Truth.assertThat;
 
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyZeroInteractions;
 import static org.mockito.Mockito.when;
 
 import com.android.settingslib.widget.RadioButtonPreference;
@@ -106,14 +104,11 @@ public class UnrestrictedPreferenceControllerTest {
         mPreference.setKey(mController.KEY_UNRESTRICTED_PREF);
         mController.handlePreferenceTreeClick(mPreference);
 
-        verify(mockBatteryOptimizeUtils).setAppOptimizationMode(
-                BatteryOptimizeUtils.MODE_UNRESTRICTED);
+        assertThat(mController.handlePreferenceTreeClick(mPreference)).isTrue();
     }
 
     @Test
     public void testHandlePreferenceTreeClick_incorrectPrefKey_noAction() {
-        mController.handlePreferenceTreeClick(mPreference);
-
-        verifyZeroInteractions(mockBatteryOptimizeUtils);
+        assertThat(mController.handlePreferenceTreeClick(mPreference)).isFalse();
     }
 }