Make WebRTC Android examples build without sourcing envsetup.sh
Make WebRTC Android examples build without sourcing envsetup.sh

The new recipes framework for configuring build explicitly sets the
GYP_DEFINES for Android builds instead of relying on the envsetup.sh script
which probably will be removed at some point in the future.

This causes our build to break since our Android examples relies on the
Android SDK being found using the ANDROID_SDK_ROOT environment variable.
A GYP variable 'android_sdk_root' exists and is set correctly by
common.gypi, which is what I'm using to pass this path correctly to these
tests.

The libjingle example is handled separately in
https://webrtc-codereview.appspot.com/11809004/

BUG=chromium:346198
TEST=Local builds using:
. build/android/envsetup.sh
unset ANDROID_SDK_ROOT
webrtc/build/gyp_webrtc
ninja -C out/Debug
ninja -C out/Release
+ trybots passing: git try --bot=android,android_rel,android_clang

R=henrike@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/11799004

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@5907 4adac7df-926f-26a2-2b94-8c16560cd09d

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