GraphicsEnvironment: Fix ANGLE system driver selection logic

When ANGLE is the system driver, we had a check that would ensure the
deferlist had been applied before allowing ANGLE as the default driver.
But rather than look at the deferlist, the check looked at the opt-in
list size.  That logic wasn't working, so the default scenario would
always give you the legacy driver.

To fix, check that the deferlist itself is not empty.

Test: atest CtsAngleIntegrationHostTestCases (results unchanged)
Test: angle_trace_test w/ ANGLE and native selected via settings
Bug: b/271862350
Change-Id: I698fae2145dbfcad8657d6f62034b18bf580b6bb