Add support for parsing 'LOCAL_TARGET_SDK_VERSION' in androidmk

Update androidmk to enable parsing of 'LOCAL_TARGET_SDK_VERSION' from
an Android.mk and translating it to target_sdk_version in an Android.bp.

Bug: 313336240
Test: m androidmk
Test: androidmk vendor/xts/gts-tests/tests/assist/Android.mk
Change-Id: I465aba3ea7d22a9978fc370a6718a2cfc43dce36