Comment more on component callback order for N+ OS

This CL attempts to clarify that just having a ContentProvider does
not necessarily mean ContentProvider#onCreate() is always called
before Application#onCreate() on Android N and later devices because
of direct boot.

Fix: 67559645
Test: make -j doc-comment-check-docs
Change-Id: I33b7cda42146333f48fb445027f5d31f3c5af5b6