Pass single-exported-file parameter

This change passes single-exported-files parameter when generates
exported_java_aconfig_library. It will generate an extra file
ExportedFlags for each class. This class is preferred to be used to
access the flags

NO_IFTTT=The mode in regular exported doesn't need to change
Test: m exported_java_aconfig_library, and use it
Bug: 392871534

Change-Id: I8766b350130f2e6c80876abcff1e1d30f55203ca