Refactor start activity methods

There were several startActivity* methods that looks similar, but not
entirely the same. Re-organized the code flow and remove some confused
methods, like setMayWait(), to make people easier to follow.

Bug: 139449647
Test: atest WmTests CtsWindowManagerDeviceTestCases
Change-Id: If1c3cc2bef9fbeecde9ec456515c9979b8286f97