bluedroid: Add option for a device-specific bluedroid

This patch adds TARGET_CUSTOM_BLUEDROID which can be used to replace
the standard bluetooth.c with a custom implementation without breaking
things for devices that assume default functionality. (Gingerbread:
566d2f2dc04f0538137e1705f898b92740db450d)

Conflicts:
	bluedroid/Android.mk

Change-Id: Icd7861054cbc284c3811ee03991da215d701798f