Add exported_java_aconfig_library

Add a rule to generate a JAR file containing code for all aconfig flags
in all packages. This is intended for apps compiled outside the
platform.

Also add a rule to include the new JAR file among the SDK artifacts.

Note: a future CL will refine the aconfig command line options used, or
filter the aconfig output: right now the JAR really contains *all*
flags; we want it to include only *exported* flags.

Bug: 311151343
Test: m sdk dist # manually verify that this generates $(gettop)/out/dist/android-flags.jar
Change-Id: I73481a9f723a0e5487cfcd2ee697873ecc4e8275