Add debug property to enable/disable ANGLE

Due to b/118375731 we suspect that loading & unloading the feature
library is causing issues with spurious segfaults or other errors
when starting other apps. Disable use of feature library by default
until issue is resolved.
Bug: 118375731
Test: manual: adb shell setprop debug.angle.enable 1
      Verify that feature library is loaded
      adb shell setprop debug.angle.enable 0
      Verify that feature library is not loaded

Change-Id: I80528acfe5138420085d24c4e7a3fc4e74903b88