GraphicsEnv: Fix isDebuggable

Update the logic used in GraphicsEnv to decide whether shared objects
can be inserted into the process.  This is used by Vulkan layers,
GLES layers, and ANGLE when deciding whether libraries from outside
packages can be used.

The new logic doesn't just use PR_GET_DUMPABLE which is no longer set
by default in platform debug builds. It also incorporates
ANDROID_DEBUGGABLE, which is defined when `debuggable` is true in
Android.bp. This happens for eng or userdebug builds of the platform.

The use of `debuggable` is the replacement for reading ro.debuggable
which can no longer be read at runtime (see b/193912100).

Tested with:
export APP_PACKAGE=<app from Play Store>
adb shell settings put global angle_debug_package org.chromium.angle
adb shell settings put global angle_gl_driver_selection_pkgs $APP_PACKAGE
adb shell settings put global angle_gl_driver_selection_values angle

Test: Released app able to load from angle_debug_package on userdebug
Test: Released app cannot use angle_debug_package on user build
Bug: b/193912100
Bug: b/253678459
Change-Id: I3dda4258e23871ee2fab2cf5ba367612e00de0e2
(cherry picked from commit 5d32e628)