aconfig: add single-exported-file parameter

This change adds a new parameter single-exported-files for the command
create-java-lib. When this parameter is true in exported mode, a new
class ExportedFlags will be generated.

Test: atest aconfig.test.java
Bug: 392871534

Change-Id: I817b1ca4fbb70082267f8ccbde4d4384ebd1bebb