Cherry-picked from e6b969f3 in master. Do not merge.

Add boot property enabling ADBD over QEMU

Emulator and system image now support ADBD communication over QEMUD pipe rather
than over a TCP port forwarding. However, emulator has to know ahead of time
(before system starts booting) whether or not the system image supports ADBD
over pipe to properly setup the communication. For that, we introduce a boot
property "ro.adb.qemu" that is readable by the emulator early enough for the
proper ADB communication setup.

Change-Id: I978489c5acf46177b520e775d745bcc78f469837