Initialialize jhp's 'enabled_' field

Otherwise, with jemalloc, which doesn't memset(0), we wrongly enable the
sampler, which makes allocations slower due to smaller TLABs.

Also move IsEnabled() to the header file as it is invoked quite a few
times while allocating TLAB.

Bug: 310655393
Test: manual testing on devices using jemalloc
Change-Id: I63e819488fb980db95080fa17400712bd8efdbe5
2 files changed