Introduce GOANDROID_ADB_FLAGS for additional flags to adb invocations.
With GOANDROID_ADG_FLAGS, the Android builders can distinguish between
emulator and device builds.

Change-Id: I11729926a523ee27f6a3795cb2cfb64a9454f0a5
Reviewed-on: https://go-review.googlesource.com/88795
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>