Replace ANDROID_SDK_HOME with ANDROID_PREFS_ROOT for metalava

Fixes warnings:
Warning: Using ANDROID_SDK_HOME for the location of the '.android' preferences location is deprecated, please use ANDROID_PREFS_ROOT instead.
Support for ANDROID_SDK_HOME is deprecated and will be removed in 6.0

Test: m android_stubs_current
Change-Id: Ie8721dcda0578c670dfc796675ba43cda16883f6
diff --git a/java/droiddoc.go b/java/droiddoc.go
index 34f017d..0becb84 100644
--- a/java/droiddoc.go
+++ b/java/droiddoc.go
@@ -1209,7 +1209,7 @@
 	rule.Command().Text("mkdir -p").Flag(homeDir.String())
 
 	cmd := rule.Command()
-	cmd.FlagWithArg("ANDROID_SDK_HOME=", homeDir.String())
+	cmd.FlagWithArg("ANDROID_PREFS_ROOT=", homeDir.String())
 
 	if ctx.Config().UseRBE() && ctx.Config().IsEnvTrue("RBE_METALAVA") {
 		rule.Remoteable(android.RemoteRuleSupports{RBE: true})
@@ -1234,7 +1234,7 @@
 			RSPFiles:             []string{implicitsRsp.String()},
 			ToolchainInputs:      []string{config.JavaCmd(ctx).String()},
 			Platform:             map[string]string{remoteexec.PoolKey: pool},
-			EnvironmentVariables: []string{"ANDROID_SDK_HOME"},
+			EnvironmentVariables: []string{"ANDROID_PREFS_ROOT"},
 		}).NoVarTemplate(ctx.Config().RBEWrapper()))
 	}