Revert "Stop writing coverage data in coverage-flush signal handler"

Bug: http://b/194128476
Bug: http://b/210012154

This reverts commit c99fe0b3c3a4cabc080f5dbd5bdf022b02b04133.

Coverage metrics dropped for ~10 of the 40 modules.  There are also
regressions in mainline when running tests on older platform builds.

Test: presubmit
Change-Id: Ifc36ca87363890347cbb83e0e1e58d443e078018
3 files changed