Make purging clear the current thread cache too.

Bug: 131362671

Test: Ran bionic unit tests.
Test: Wrote a test that did a purge and verified that the tcache->ncached
Test: number went from 3 to 0. Also verified that mallinfo reflects the
Test: cached entries being flushed.
Change-Id: I64e35618fa3b523cf29bdaceedef676abe440dd3
(cherry picked from commit 0f710fd59346312b4e351e9d3c956bc804ff02b2)