Added protections to RenderScript VTS class teardown.

If RenderScript HIDL server does not exist or cannot be opened,
currently it results in a nullptr dereference when the VTS test is
torn down. This CL adds a check before the cleanup.

Test: mm -j40
Bug: 63711002
Change-Id: I11a226a78b31444e7b198b2ff1dfa3794963caa0
1 file changed