target_sdk_version replacement should respect REL branches

Currently, target_sdk_version of MTS tests were being set to 10000 even
in release branches. This should only happen in active branches.

Test: Unit test
Test: aapt2 on CtsContentTestCases in udc-dev and verified
targetSdkVersion is 34
Bug: 279507428

Change-Id: Ib79318febc0f6e771b3679c0281a97357930ec56