Assert the errors size to 1 for case SetDisplayBrightness

null pointer dereference crash will be triggered if vector errors
is emptry when accessing errors[0].errorCode, so change EXPECT_EQ
to ASSERT_EQ to avoid this issue.

Test: run vts -m VtsHalGraphicsComposer3_TargetTest

Signed-off-by: Liu, Kai1 <kai1.liu@intel.com>
(cherry picked from https://android-review.googlesource.com/q/commit:aec33e9a41e4bccfd1834e1ab03f785eea4f0b52)
Merged-In: I3b30d491ef886b98e462447e6de78828e7822fca
Change-Id: I3b30d491ef886b98e462447e6de78828e7822fca