Perhaps relevant for your initial problem: Can you charge the tablet form a power jack? If so you can try to use a combined notebook power supply with additional USB power out. E.g. http://www.conrad.biz/ce/de/product/515004/VOLTCRAFT-NPS-90-USB-Notebook-Netzteil-Laptop-Netzteil-BASIC-LINE-12V14V15V16V18V185V19V195V20V21V22V-/ So you can drive the tablet by power jack and the Arduino via USB - if 5 V/1 A via USB fits your need.
If you ran both thru diodes, the source with the higher voltage would reverse bias the lower voltage source's diode and keep that source turned off.