Tweaks to Activity up navigation

Be more explicit in docs about getParentActivityIntent and guard against
calls when a parent has not been declared in the manifest.

Move automatic up navigation to happen after dispatch of the menu
selection event of id android.R.id.home to fragments. (Last.)

Fixes bug 6305357

Change-Id: I944e5c40774121f9a28250d8d98da6aa646f9357