Wait for device to charge between test cases

This change mitigates the problem where the device runs out of battery
when running a stress test for days while charging via USB.

Fix: 136149280
Test: build_and_run_benchmark.sh inference-stress
Test: build_and_run_benchmark.sh model-loading-stress
Merged-In: I33c8eb4c818b769dbe98ff2b0068acad130cbc4a
Change-Id: I33c8eb4c818b769dbe98ff2b0068acad130cbc4a
(cherry picked from commit 5f056ac3777d850cbeb12b40965a4d8b65ccff08)
3 files changed