Update ActivityManager DeviceConfig flag definitions.

1. Remove the interface from DeviceConfig.
2. Add a string constant for the namespace to DeviceConfig, including
them in the System API.
3. Add string constants for flag names to ActivityManagerConstants, and
AppCompactor. (These are removed from the System API).

Bug: 126411407
Test: atest AppCompactorTest
      atest FrameworksCoreTests:DeviceConfigTest

Change-Id: I3167a4267e2ea0277f19f7d5211ab92e2b1f6285