apex.custom_sign_tool property

A new property indicates a CLI tool to sign the APEX contents. The value
is stored in apexkeys.txt so that releasetool (sign_target_files_apks)
can use it to invoke the tool to re-sign the apex contents.

Bug: 193504286
Test: m out/soong/apexkeys.txt
      com.android.virt.apex line has sign_tool value
Change-Id: Ifd472049b75b5b87c1ed320f5e1190ff65ed54f2

apex/apex_test.go

@@ -7683,6 +7683,28 @@ func TestApexKeysTxt(t *testing.T) {
 			name: "myapex",
 			key: "myapex.key",
 			updatable: false,
+			custom_sign_tool: "sign_myapex",
+		}
+
+		apex_key {
+			name: "myapex.key",
+			public_key: "testkey.avbpubkey",
+			private_key: "testkey.pem",
+		}
+	`)
+
+	apexKeysText := ctx.SingletonForTests("apex_keys_text")
+	content := apexKeysText.MaybeDescription("apexkeys.txt").BuildParams.Args["content"]
+	ensureContains(t, content, `name="myapex.apex" public_key="vendor/foo/devkeys/testkey.avbpubkey" private_key="vendor/foo/devkeys/testkey.pem" container_certificate="vendor/foo/devkeys/test.x509.pem" container_private_key="vendor/foo/devkeys/test.pk8" partition="system_ext" sign_tool="sign_myapex"`)
+}
+
+func TestApexKeysTxtOverrides(t *testing.T) {
+	ctx := testApex(t, `
+		apex {
+			name: "myapex",
+			key: "myapex.key",
+			updatable: false,
+			custom_sign_tool: "sign_myapex",
 		}
 
 		apex_key {