Allow per-app opt in to use system ANGLE.

Previously the per-app opt in only works for ANGLE apk, this patch
refactors the logic to allow per-app opt in to work for system ANGLE.
This patch checks whether system ANGLE is supported and if it presents,
uses it if the ANGLE apk setup fails.

With this patch, the per-app opt in will function this way:

1) When full ANGLE apk is installed
Regardless of whether system ANGLE is present, the "angle" option will
use ANGLE apk, the "native" option will switch to native GLES drivers
specified by ro.hardware.egl, the "default" option will use whatever the
default graphics driver is loaded.

2) When full ANGLE apk is not installed and system ANGLE is present
The "angle" option will use system ANGLE, the "native" option will
switch to native GLES drivers specified by ro.hardware.egl, the
"default" option will use whatever the default graphics driver is
loaded.

3) When full ANGLE apk is not installed and system ANGLE doesn't exist
The per-app option will not function at all.

Minor: clean up unused and unnecessary variables.
Minor: Avoid sending toast when ANGLE apk doesn't present

Bug: b/283858001
Test: test with camera
Test: atest CtsAngleIntegrationHostTestCases
Change-Id: I83b3c2ccbe7cb38f71418afde4254f48153ab6b3