Bp2build support for sdk_version and java_version.

This CL adds java_version and sdk_version support to bp2build
converters for
   - java library
   - java binary
   - android library
   - android binary
   - android library import

Although java import doesn't support java_version and sdk_version, the
neverlink java_library wrapper around a java_import must specify a
sdk_version when targetting a device. "none" is used by convention.

Change-Id: I22a69dea2e351858368df69ed6a703b568d613ea
Bug: 215230098
Test: Presubmits
diff --git a/bp2build/java_plugin_conversion_test.go b/bp2build/java_plugin_conversion_test.go
index 8c6337b..f2b6f20 100644
--- a/bp2build/java_plugin_conversion_test.go
+++ b/bp2build/java_plugin_conversion_test.go
@@ -67,7 +67,7 @@
         "a.java",
         "b.java",
     ]`,
-				"javacopts": `["-source 1.7 -target 1.7"]`,
+				"java_version": `"7"`,
 			}),
 		},
 	})