Disable activity leak detection by default for userdebug

Causes too many GCs and related slowdowns.
Verified that assistant launch from holding down home button is now
faster than N.

Test: make and flash
Bug: 62769566

Change-Id: Ib0c1f7a45831b241d3376d1e56db3c6937913b1b
(cherry picked from commit ca8c1051)