Introduce BazelStringOrLabelFromProp.

Soong supports string properties, but they are overloaded, and can mean
one of three things:

* path reference
* module reference
* string literal

Bazel has different types: label and string attributes. Thus there needs
to be a way to categorize them correctly in bp2build.

This CL introduces a new function to be used on properties like
apex_key.private_key / apex_key.public_key, as well as
android_app.certificate / apex.certificate.

It is important to disambiguate the prop betenn a string literal
attribute or file/rule target label attribute, so this functions does
just that.  The new attributes are then further handled by their
respective macros (apex_key, android_binary, apex).

Bug: 253557437
Fixes: 253557437
Test: presubmits, new tests

Change-Id: Id8111cdd60d3aabcae7d17fe9da84d0ee3966023
diff --git a/bp2build/apex_key_conversion_test.go b/bp2build/apex_key_conversion_test.go
index 79b1d89..f9a68c9 100644
--- a/bp2build/apex_key_conversion_test.go
+++ b/bp2build/apex_key_conversion_test.go
@@ -30,12 +30,37 @@
 	ctx.RegisterModuleType("filegroup", android.FileGroupFactory)
 }
 
-func TestApexKeySimple_KeysAreSrcFiles(t *testing.T) {
+func TestApexKeySimple_KeysAreSrcFilesInSameDir(t *testing.T) {
 	runApexKeyTestCase(t, Bp2buildTestCase{
-		Description:                "apex key - keys are src files, use key_name attributes",
+		Description:                "apex key - keys are src files, use key attributes",
 		ModuleTypeUnderTest:        "apex_key",
 		ModuleTypeUnderTestFactory: apex.ApexKeyFactory,
-		Filesystem:                 map[string]string{},
+		Filesystem: map[string]string{
+			"com.android.apogee.avbpubkey": "",
+			"com.android.apogee.pem":       "",
+		},
+		Blueprint: `
+apex_key {
+        name: "com.android.apogee.key",
+        public_key: "com.android.apogee.avbpubkey",
+        private_key: "com.android.apogee.pem",
+}
+`,
+		ExpectedBazelTargets: []string{MakeBazelTargetNoRestrictions("apex_key", "com.android.apogee.key", AttrNameToString{
+			"private_key": `"com.android.apogee.pem"`,
+			"public_key":  `"com.android.apogee.avbpubkey"`,
+		}),
+		}})
+}
+
+func TestApexKeySimple_KeysAreSrcFilesNotInDir(t *testing.T) {
+	runApexKeyTestCase(t, Bp2buildTestCase{
+		Description:                "apex key - keys are not src or module, use key_name attributes",
+		ModuleTypeUnderTest:        "apex_key",
+		ModuleTypeUnderTestFactory: apex.ApexKeyFactory,
+		Filesystem:                 map[string]string{
+			// deliberately left empty
+		},
 		Blueprint: `
 apex_key {
         name: "com.android.apogee.key",