Use ThreadPoolExecutor to run external tools for signing/verification to
avoid unnecessary IO waiting which makes the tool extremely slow and
tests depending on it fail.
Local test shows `sign_virt_apex_test` running time reduced 15s->4s.
Bug: 229721225
Test: atest --host sign_virt_apex_test
Test: atest MicrodroidHostTestCases
Test: presubmit
Change-Id: I2b1ebe9f74e06256a049565a936c3b604a1775ce