Notify split controller about activity configuration change

Track activity configuration changes, so that split controller has
a chance to apply metrics-based rules like launching a placeholder.

Not using ComponentCallbacks here, since they are only triggered for
Application configuration changes, which might be problematic if
there are multiple running activities with different sizes. They are
also usually triggered before individual activity resources are
updated.

Also moved post-creation actions to onActivityPostCreated(), since
onActivityCreated() is called from the base implementation in
Activity#onCreate() and there is still a chance that app may perform
some actions after calling super in its override.

Bug: 194538838
Test: Configure a placeholder, launch in a small size, enlarge.
Change-Id: Ied55d5f920890fa554cdd66419235d61ab03b440