llkd: default enabled for userdebug

If LLK_ENABLE_DEFAULT is false, then check "ro.llk.enable" for "eng",
also the default value if not set, and then check if userdebug build
to establish a default of true for enable.  Same for
ro.khungtask.enable.

Test: llkd_unit_test report eng status on "userdebug" or "user" builds
Bug: 33808187
Bug: 72838192
Change-Id: I2adb23c7629dccaa2856c50bccbf4e363703c82c