The ANDROID_SERIAL variable is used adb and gradle, as well as other parts of
our scripts. This leaves the DEVICE_SERIAL checks in place as a fallback since
some of our hardware CI workers still set that variable to indicate device.
Going forward, ANDROID_SERIAL should be prefered for compatibility with android
ecosystem.
Differential Revision: https://phabricator.services.mozilla.com/D237288
26ae33dd08