Fix UnminimizeAppFromTaskbarTest by using gmail instead of browser app.

The browser app consistently fails on setup because it can't find the
minimize button, so use Gmail instead which is by default always on the
taskbar.

Test: atest WMShellFlickerTestsDesktopMode:com.android.wm.shell.functional.UnminimizeAppFromTaskbarTest
Fixes: 400419745
Flag: EXEMPT flicker test fix

Change-Id: I4882b4d1796507406d6babab667b80f0287fd59b