Add a fake library that contains the minimum DeviceConfig that aconfig code needs to compile.

The aconfig-generated code in the framework is added with srcjars. To
get those srcjars, we still need a java_aconfig_library.  This class is
there to allow that to compile, even though those compiled .jar files
are not actually used.

Bug: 293195086
Test: m
Change-Id: I34c21925fbc5912a34fea3e7796d5e0c77c9a4af