Fix a missing case in unloading system driver

When app requests to use ANGLE on devices without an
ANGLE apk installed, but the device has preloaded native
GLES driver, the current should_unload_system_driver()
function does not recognize that we should unload the preloaded
GLES driver, and load the ANGLE driver from system parition.

This CL adds this missed check in should_unload_system_driver()

Bug: b/333483712
Test: atest android.angle.cts.CtsAngleDeveloperOptionHostTest
      Result: https://android-build.corp.google.com/abtd/run/L47700030003349338/

Change-Id: I4460eed9f05ee0e19ddb6e4d2a3375701414c458