User can abort a running benchmark

[anhappdev]

With #718 a long-running benchmark will be aborted after 10 minutes. But it doesn't work for the Offline scenario (see discussion at mlcommons/inference#1397).

Another way to stop a running benchmark is to use:

/// \brief Aborts the running test.
/// \details This function will stop issueing new samples to the SUT. StartTest
/// will return after the current inference finishes. Since StartTest is a
/// blocking function, this function can only be called in another thread.
void AbortTest();

https://github.com/mlcommons/inference/blob/da9b4d54bd2bd2ae26316f75987d71103122e3ca/loadgen/loadgen.h#L79-L83

The app should have a button in the progress screen so that users can abort the running benchmark.

[anhappdev]

In my testing, AbortTest() did not work as expected. When calling AbortTest(), the app just hangs forever and StartTest() never finished.

The issue was discussed in https://github.com/mlcommons/mobile_app/issues/289, but there was no solution.

For reference, here is the branch with AbortTest() implemented: 749/abort-running-benchmark

[freedomtan]

@anhappdev to create an issue for loadgen