Export ReleaseToolsExtensionDir to soong

This directory contains a releasetools.py file which will be packaged
into target_files.zip

Also export AbOtaPostInstallConfig. This was missed when adding support
for this in https://r.android.com/3493530

Test: Built soong target_files.zip locally
Bug: 388633394

Change-Id: I1f8385b743f319dc28af8ac0460bbc13b10dbb9f