Check original DeviceConfig flag in allowlist for overrides

Android 16 limits the DeviceConfig flags that can be written by
the shell user to those that have been allowlisted. When a flag
is modified through the DeviceConfig#setLocalOverride method,
the requested flag is placed in the device_config_overrides
namespace, and the flag in this new namespace is used during the
allowlist check. Because of this new namespace, flags that were
previously allowlisted would fail the check without a new entry
in the allowlist using the device_config_overrides namespace.
This commit checks for this override namespace, obtains the
original flag, and uses that flag to verify whether the shell
user can modify it..

Bug: 388608113
Flag: android.security.protect_device_config_flags
Test: atest DeviceConfigApiTests
Test: atest android.provider.SettingsProviderTest
Change-Id: Ia5853fc1399bdb3b4767ae65badf0298acda31ea