Create ANGLE namespace in GraphicsEnvironment

This commit does the following:

* Create a new Android Setting for use during development,
  "angle_enabled_app", which can be used to load ANGLE
  only for specified applications.

  Usage:
    adb shell settings put global angle_enabled_app <package-name>

* When the package name condition is met, build a namespace
  that points to the ANGLE APK.  The namespace will be used
  by the EGL Loader to find ANGLE libs.

* The APK may be the pre-installed version in /system/app/ANGLE,
  or it may be an updated version from the Play Store, which will
  reside somewhere under /data/app/*.

Test: Manual

Change-Id: Ia0475cf80cf5c2589e52fdb3c8f3672696357923