Bug 874822 - Allow gaiatest to recover from B2G crashes

This fixes the problem described in bug 874822, in which a B2G crash causes gaiatest to be unable to restart B2G and proceed successfully to the next step.

It does this by caching the value of 'is_android_build' on the testvars object, which persists between tests, so that gaiatest's setUp method can determine if 'is_android_build' without an active Marionette session.

This patch causes the test case in bug 874822 to pass.