Update seinfo for sdk sandbox

Before we start the sdk sandbox process, add a flag to seInfo,
if a DeviceConfig flag is present, which causes libselinux to apply
sdk_sandbox_next sepolicy for the process.

This enables some testers to test out the set of restrictions
we're planning for the next SDK version.
sdk_sandbox_next is not the final set of restrictions of the next SDK
version.

Bug: b/270148964
Test: using the manual test app and adb shell ps -Z with device_config
toggle
atest ActivityManagerServiceTest
Change-Id: I126a3b4104b4fd21e71085858c6bbc3591e38470

Change-Id: I4beda06b592f94ce44d9d504f4ff032f303ae3e4