ahat-tests: don't write to src directory during build

The art script writes to $PWD/android-data$$ by default, which is
in the source tree and may not be writeable.  It also causes the
timestamp of the top level source dir to change, which can cause
build globs to rerun on the next build and breaks unnecessary
rebuild checks.

Test: m checkbuild
Bug: 68105950
Change-Id: I75ddb6519b1c94132635517a6b65ea24cad25d91