Handle boot jars on /system_ext correctly.

When Soong looks for boot jars among all modules, it applies certain
constraints to each module that looks like a boot jar (e.g. that it
comes from the right apex or platform). Previously these constraints did
not handle boot jars on /system_ext correctly (they were handled like
apex jars, while they should be handled like platform jars).

Bug: 154976937
Test: m nothing (the modified Soong test would fail previously)
Change-Id: I0746a2fd276ab5ef0400340c5b61cf26c2570e5a

java/dexpreopt_bootjars_test.go

@@ -35,6 +35,7 @@ func testDexpreoptBoot(t *testing.T, ruleFile string, expectedInputs, expectedOu
 			name: "bar",
 			srcs: ["b.java"],
 			installable: true,
+			system_ext_specific: true,
 		}
 
 		dex_import {
@@ -47,7 +48,7 @@ func testDexpreoptBoot(t *testing.T, ruleFile string, expectedInputs, expectedOu
 		prepareForJavaTest,
 		PrepareForTestWithJavaSdkLibraryFiles,
 		FixtureWithLastReleaseApis("foo"),
-		dexpreopt.FixtureSetBootJars("platform:foo", "platform:bar", "platform:baz"),
+		dexpreopt.FixtureSetBootJars("platform:foo", "system_ext:bar", "platform:baz"),
 	).RunTestWithBp(t, bp)
 
 	dexpreoptBootJars := result.SingletonForTests("dex_bootjars")