Improve LocaleManagerBackupRestoreTest

Resolve the mismatch between the UID and user ID that occurs when the
Headless System User Model (HSUM) configuration is enabled.

Flag: TEST_ONLY
Bug: 396779832
     396941211
     396968635
     389876110
     389876352
     389876069
Test: atest FrameworksServicesTests_android_server_locales:com.android.server.locales.LocaleManagerBackupRestoreTest -- --abi x86_64
Change-Id: I2c5ad737dc2937012457e0e01a36a842fdb2d03d