[Signin][Android] Add waitForIdleSync and asserts to ManageSyncSettingsTest

This test has been flaky, so this CL adds waitForIdleSync to wait for
UI to settle. It also adds an assert to verify that "Sync everything"
toggle is turned off.

Bug: 1059116
Change-Id: I3955eaa187c5ce9f57ac715fc416e9b88a5ac461
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108555Reviewed-by: Alice Wang <aliceywang@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756296}

chrome/android/javatests/src/org/chromium/chrome/browser/sync/ManageSyncSettingsTest.java

@@ -111,8 +111,12 @@ public class ManageSyncSettingsTest {
         ChromeSwitchPreference syncEverything = getSyncEverything(fragment);
         Map<Integer, CheckBoxPreference> dataTypes = getDataTypes(fragment);
 
+        InstrumentationRegistry.getInstrumentation().waitForIdleSync();
         assertSyncOnState(fragment);
         mSyncTestRule.togglePreference(syncEverything);
+
+        InstrumentationRegistry.getInstrumentation().waitForIdleSync();
+        Assert.assertFalse(syncEverything.isChecked());
         for (CheckBoxPreference dataType : dataTypes.values()) {
             Assert.assertTrue(dataType.isChecked());
             Assert.assertTrue(dataType.isEnabled());