Add a permission test to ensure enforcement of SYSTEM_CAMERA permissions.

Add a permission test to ensure a process not having SYSTEM_CAMERA
permissions cannot discover / connect camera ids having the
SYSTEM_CAMERA capability.

Bug: 138135360

Test: atest Camera2PermissionTest.java#testSystemCameraDiscovery
Test: hardcode all cameras as system cameras;
      atest Camera2PermissionTest.java#testSystemCameraDiscovery

Change-Id: I2f56fa4175649829cf2ad65a82b4d8b42b3034a9
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>