Setup java paths in soong_ui

This way config.mk no longer needs to check which java is in PATH and
fix it. It'll be consistent for all build steps under soong_ui.

Also unify handling of ANDROID_JAVA_HOME / JAVA_HOME with
OVERRIDE_ANDROID_JAVA_HOME / EXPERIMENTAL_USE_OPENJDK9.

Test: m nothing
Test: build/soong/soong_ui.bash --make-mode nothing (w/o envsetup.sh)
Test: aosp_arm ninja files are the same before/after
Test: before/after ninja files match with OVERRIDE_ANDROID_JAVA_HOME
Test: before/after ninja files match with EXPERIMENTAL_USE_OPENJDK9
Change-Id: Icdb65093d9c346524074de239a4f895e4230a24d

ui/build/util.go

@@ -24,6 +24,14 @@ import (
 	"unsafe"
 )
 
+func absPath(ctx Context, p string) string {
+	ret, err := filepath.Abs(p)
+	if err != nil {
+		ctx.Fatalf("Failed to get absolute path: %v", err)
+	}
+	return ret
+}
+
 // indexList finds the index of a string in a []string
 func indexList(s string, list []string) int {
 	for i, l := range list {