Add nullable and nonnullable annotations to DeviceConfig.

Also test coverage for those methods.

Bug: 126360191
Bug: 126699440
Bug: 126702152
Bug: 126702160
Test: atest FrameworksCoreTests:DeviceConfigTest
Change-Id: I9ecb1be76c88d0e61b8f68375d6659249449c564
4 files changed