Android: rename android_build_type gyp variable.
Android: rename android_build_type gyp variable.

Following Chromium r187556 this variable has been renamed to
android_webview_build to better describe what it does.

Contributed by torne@chromium.org (https://webrtc-codereview.appspot.com/1195006/).
Review URL: https://webrtc-codereview.appspot.com/1214005

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 95a8ddd272f2066548772602a863d8e9d3293a6f

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc