It addresses the ScreenRecoveryUITest failures on gce targets which
don't have any graphics backend. Probing for all backend devices in
tests could work, but would duplicate codes. This CL relies on the
result of gr_init().
As a side effect, it may give false negatives if gr_init() is supposed
to work but silently broken. But such issues are beyond
ScreenRecoveryUITest's concern, which should be captured by the tests
for minui or graphics backends instead.
Fixes: 79616356
Test: Run recovery_unit_test on marlin.
Test: Run recovery_unit_test on gce.
Change-Id: I121aacc61c8a614447509506057ecfd8d86163e4
51f16ec76d