VCN: Remove hidden APIs for data directory and Settings constants

This patch removes following hidden API usages for mainling VCN:
- Environment#getDataSystemDirectory()
- Settings.Global.MOBILE_DATA

Bug: 288465472
Test: atest FrameworksVcnTest && atest CtsVcnTestCases
Flag: EXEMPT pure refactoring
Change-Id: I66a605c4f460d456ac4349ffb67ecd5e7acc7438