adb: Allow configuring the adb shell with a persistent property.

To set to bash: setprop persist.sys.adb.shell /system/xbin/bash

Change-Id: I24e65f4592aaf23356c13db02c9869076df3b841
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_system_core into temp