Build java_sdk_library stubs with language level 8

This is in line with the main android stubs (android.jar). I discovered
the difference since we want to merge some java_sdk_libraries into the
android.jar files, but I believe the rationale for compiling it with
version 8 applies to all stubs (we don't want level 9-language features
because some tooling may be stuck with version 8).

Bug: 144149403
Test: presubmit (checkbuild)
Change-Id: I9439ed516dda68ef516ff08d1c6045de3d3c1c0e
Merged-In: I9439ed516dda68ef516ff08d1c6045de3d3c1c0e
(cherry picked from commit cf4dd4c1df5ca2d91e6fa097657826eced116d21)
diff --git a/java/sdk_library.go b/java/sdk_library.go
index 67b0bd6..ee62390 100644
--- a/java/sdk_library.go
+++ b/java/sdk_library.go
@@ -1113,7 +1113,9 @@
 	props.Product_variables.Pdk.Enabled = proptools.BoolPtr(false)
 	props.Openjdk9.Srcs = module.properties.Openjdk9.Srcs
 	props.Openjdk9.Javacflags = module.properties.Openjdk9.Javacflags
-	props.Java_version = module.properties.Java_version
+	// We compile the stubs for 1.8 in line with the main android.jar stubs, and potential
+	// interop with older developer tools that don't support 1.9.
+	props.Java_version = proptools.StringPtr("1.8")
 	if module.deviceProperties.Compile_dex != nil {
 		props.Compile_dex = module.deviceProperties.Compile_dex
 	}