Send coverage flush signal to only processes that handle signal 37.

Bug: http://b/160401633

Also:
- fix regexp to find clang prebuilt version to allow a suffix.
- call `adb root` before any operation.

Test: acov-llvm.py flush
Change-Id: I9287610ee2ccfec165b0a0bc2e3b24e0eb84ca31
1 file changed