java_sdk_library: Improve consistency with ..._import

The scopePaths struct is used by both java_sdk_library and its prebuilt
but was not populated in the same way. This change addresses those
discrepancies in preparation for a follow up change which will allow
access to some of those fields through OutputFileProvider.

Changes:
* Document the scopePaths field and struct.
* Switch those fields that may not be fully populated from Paths to
  OptionalPath to make that 100% clear and protect against unchecked
  use.
* Switch java_sdk_library_import to use the dependency extraction
  mechanism driven by the dependency tag. This should actually have
  been part of the change that added that mechanism.
* Only create prebuilt_stubs_sources if sources have been provided.
* Add dependencies from java_sdk_library_import on its stubs source
  child modules if sources have been provided. That will ensure the
  stubsSrcJar field is updated.
* Updates current/removedApiFilePath if provided for the scope in
  java_sdk_library_import.
* Extracts ApiStubsSrcProvider from ApiStubsProvider to allow it to
  be implemented by PrebuiltStubsSources so that it can provide access
  to the stubs src jar that it creates.

Test: m nothing
Bug: 148080325
Bug: 155164730
Change-Id: Ic5bf884b2b1e79841843e7c3b4642796ecd49f5d

java/java_test.go

@@ -575,6 +575,7 @@ func TestJavaSdkLibraryImport(t *testing.T) {
 			},
 			test: {
 				jars: ["c.jar"],
+				stub_srcs: ["c.java"],
 			},
 		}
 		`)