Set targetSdkVersion to 10000 for MTS tests targeting current

MTS tests built on unstable branches (e.g. git_master) should be testable on
old system images (e.g. S). However, they run into an error during installation
on older images:
`Requires development platform $<current_codename>, but this is a
release platform`

This CL fixes this issue by updating the target_sdk_version of MTS test apps
targeting current to the magical sdk_version 10000

Bug: 227473065
Test: go build ./java
Test: TH
Change-Id: Ic0358a48a19dc239defbb4ee8ec99225cce75584