Avoid unloading ANGLE.

Previously when ANGLE is the default GLES driver and preloaded, by
setting an application to use ANGLE when ANGLE apk doesn't present, the
system ANGLE should be used. However, the loader will unload the default
driver and load ANGLE. And hence when ANGLE is the default GLES driver,
it will be unloaded and then reloaded. This patch makes sure the loader
skips unloading and immediately return in this case.

Minor: Only unload the drivers when there are preloaded drivers.

Bug: b/283858001
Test: verified with camera
Test: verified by forcing GLES driver preloading
Change-Id: I82b6408405ef7c507e50ab259204bdce95fda110