Allow "shortcut manager" icons to be returned to apps

... and client apps to read them.

A full path looks like this:
/data/system_ce/[user-id]/shortcut_service/bitmaps/[creator-app-package]/[timestamp].png

System server will:
- Create/delete the directories.
- Write/remove PNG files in them.
- Open the PNG files and return file descriptors to client apps

Client apps will:
- Receive file descriptors and read from them.

Bug 27548047

Change-Id: I3d9ac6ab0c92b2953b84c3c5aabe1f653e6bea6b