Support caching downloaded test artifacts.

Cross-branch testing is mainly used to test platform changes with
a xTS on test-dev branches. Since testing multiple xTS builds is
rare, only one build is cached at a time.

Bug: 390161000
Test: atest_unittests & atest-dev CtsGestureTestCases --test_build_target test_suites_arm64 --test_build_id 13626066
Change-Id: I1d7eba53c39bd4a38055cf60aaf23729ceda1998
3 files changed