Move native coverage output directory.

Native coverage will be written to /data/local/tmp instead of
/data/misc/trace, allowing coverage tests to run on non-coverage builds.

Test: atest libunwindstack_unit_test --experimental-coverage
Test: m droid && acloud create --local-image
Bug: 332390317
Change-Id: I5840cef04a6e0dc3c749c53121176e1ea42029a0