Switching from mDevice.pressHome() to mLauncher.pressHome()

Using mDevice.pressHome() was a workaround for the shifted-search bug.

Change-Id: I740db2109af531c55f8ab59a42998bdc329fb72e
Tests: affected tests

tests/src/com/android/launcher3/ui/AbstractLauncherUiTest.java

@@ -123,7 +123,7 @@ public abstract class AbstractLauncherUiTest {
                 public void evaluate() throws Throwable {
                     try {
                         // Create launcher activity if necessary and bring it to the front.
-                        mDevice.pressHome();
+                        mLauncher.pressHome();
                         waitForLauncherCondition("Launcher activity wasn't created",
                                 launcher -> launcher != null);