docs: move USB installation info into the OEM USB Drivers document because

the Google USB Driver is deprecated, so consolidating the USB info makes more sense
for the common case of needing an OEM driver
A few changes to the device setup doc also; such as removing the "unknown sources"
step, because that doesn't apply to adb installs.

Change-Id: Ice78c997cbb147865ead78fb037d0b6b64db75bd