Make MainActivity the default activity for AOSP.

This also finishes moving all of the old tests over from
Dialtacts to Main. The only 2 tests that fail are:
 - searchBoxClosedWhenHandlingNotificationIntent
 - openSearchFragment_fabGoneInRegularSearchAfterRotation

I will fix these in a follow up CL.

Bug: 72525887
Test: dialer_aosp build target launches MainActivity, TAP
PiperOrigin-RevId: 189408987
Change-Id: Ice30191d95c19c62bcd89503e89f66440f30beac