chore: point android-dts-generator submodule at main (#1975)

The android-dts-generator repo renamed its default branch from master
to main and deleted master, so the submodule's `branch = master` no
longer resolves and `git submodule update --remote` fails to find it.
Point it at the branch that actually exists.

Author: adrian-niculescu

.gitmodules

@@ -7,4 +7,4 @@
 [submodule "test-app/build-tools/android-dts-generator"]
 	path = test-app/build-tools/android-dts-generator
 	url = https://github.com/NativeScript/android-dts-generator.git
-	branch = master
+	branch = main