Clear all StaticLayout.Builder fields on pool recycle

When returning a StaticLayout.Builder object to the pool, it is
important to clear everything out. If not, then the object may have
stale state from a previous layout operation. This was causing CTS
test failures and probably other mischief.

Bug: 20636917
Change-Id: I76fb20076ee954fcb476131a8fbe8833d72fbc0d