Generate "exportable" stubs library in java_sdk_library

Given that now the droidstubs module generates the "exportable" stubs,
this change modifies java_sdk_library to generate the java_library
modules that compiles the "exportable" stubs per api scope.
The detailed naming scheme of the generated modules are available in the
bug linked in the footer.

Similar to the from-text java_api_library vs from-source java_library
static lib selection for the "everything" stubs, the "exportable" stubs
module can also toggle between the stubs java_api_library and the
java_library module. However, given that the "exportable" stubs
generation has not been implemented for from-text stubs, the module
always default to depend on the from-source "exportable" stubs compiling
java_library module.

Test: go test ./java
Bug: 315495926
Change-Id: I5798312c1338c55625b2030da728b056385171a4