Make sure config change items are executed in the order dispatched.

In the previous implementation a batch of process/activity config
changes would effectively be executed out of order. When the server
would dispatch changes in config in quick succession the config change
items would update the pending configs first through the preexecute()
calls and then apply the activity config before the process config
is applied even though the process config was dispatched before the activity
config change item. See b/148639784 for more detail.

Fixes: 148639784

Test: ActivityThreadTest#testHandleActivityConfigurationChanged_EnsureUpdatesProcessedInOrder
Test: ActivityThreadTest#testHandleActivityConfigurationChanged_SkipWhenNewerConfigurationPending

Change-Id: I3c926076ac8dba73eb0471c7bc91313df519cf92