Before this change, if targetSdkVersion wasn't set in the
AndroidManifest.xml, we'd set it to the sdk_version from the Android.bp.
But there are cases where you want to compile against a later SDK, but
target an earlier one (especially if you depend on libraries that need
to be compiled against more recent SDKs, like androidx).
Test: build APK with different target_sdk_version.
Change-Id: Iaed36b522955a374a049ef331158cc8fc5798ad2