EM_VALUE: add setting

Adding EM_VALUE secure setting and config

Test: B&R tests
Test: Multi-user tests
Test: atest SettingsBackupTest
Test: atest SettingsProviderTest
Test: atest CtsPackageManagerHostTestCases_cts_readablesettingsfieldstest

Bug: 378407278
Flag: com.google.android.settings.flags.em_value
Change-Id: I6bf80719f4cb50d4ce249538edaf71b123dc5e6b