Return default value if fingerprint doesn't match

This change includes

1. Return default value if fingerprint doesn't match
2. Remove get flag value by string from PlatformAconfigPackageInternal,
   since it is not used by generated code.

Test: atest aconfig_storage_read_unit aconfig_storage_read_functional
aconfig_storage_file.test.java

Change-Id: I5ee221a19e2199700defa9121f5cae097a894d02