Add InstantTaskExecutorRule to androidTest

without that the getValue() throws "java.lang.IllegalStateException: Cannot
invoke observeForever on a background thread"

Signed-off-by: Geoffrey Métais <geoffrey.metais@gmail.com>
1 job for master in 3 minutes and 49 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #252526
android debian
continuous-app-build

00:03:49