Add support for external dock batteries

Allows the external battery status to be read via framework.

Change-Id: I1340071cecab8f1733e58e4f3ba0d304f861f311
frameworks: base: add storage_sd_dock_card string

* For Asus dock sdcards.

Change-Id: Ibe6336d4073e8e4f6c357b47bd980526a6789040