Correctly evaluating the caller as the non-top activity

The activity currently being started is irrelevant, instead
we should take the caller activity as the non-top activity
while starting activities with FLAG_ACTIVITY_PREVIOUS_IS_TOP
flag.

Bug: 122582732
Test: atest ActivityStarterTests
Change-Id: I548ab1006cfacb918f2a1c8df9570916f0b1c9dc