Allow ANGLE developer option toggle UI enabled if ANGLE is enabled.

Previously the ANGLE developer option toggle UI was disabled by default
and a user would need to use adb command to set the debug property to
true to enable it, and every reboot would reset the debug property to
false again and hence disable the toggle UI again. However, since ANGLE
is still in experiment, we would like to make sure it's not easy to
enable but easy to disable. This patch keeps ANGLE toggle UI enabled
when ANGLE is enabled so that users can easily disable. Once ANGLE is
disabled, if the debug property is not set to true, the toggle UI will
remain disabled.

Bug: b/293503000
Test: atest -c GraphicsDriverEnableAngleAsSystemDriverControllerJUnitTest
Test: atest SettingsRoboTests:GraphicsDriverEnableAngleAsSystemDriverControllerTest
Change-Id: I2e63431cd0d8cf4712fc278646627fbf34a2b542