adbd: optionally use qemud pipe without ro.kernel.qemu

adbd currently decides to use the QEMUD pipe if ro.kernel.qemu=1, as set
for ranchu. The Android container in Chrome OS doesn't have that
property set and it can't be set to 1 because it's used as equivalent to
"runs inside an emulator" throughout Android and changes the way
graphics are handled, whether Bluetooth is supported, etc., behaviour
that we do not want to trigger in Chrome OS.

adbd now also checks service.adb.transport to decide whether to use the
QEMUD (goldfish) pipe.
adbd still first checks for ro.kernel.qemu to preserve existing
behaviour and will still fallback to TCP if it can't use Goldfish.

Bug: 38497992
Test: tested by jmgao@ -thanks!- on aosp_angler, adb still works.

Change-Id: I8370704145ae7301ac7aeef81c5cbd94cfcb7fd7