init: Add gentle_kill service property

If a service specifies gentle_kill, attempt to stop it will send SIGTERM
instead of SIGKILL. After 200ms, it will issue a SIGKILL.

Bug: 249043036
Test: atest CtsInitTestCases:init#GentleKill
      Added in next patch
Change-Id: Ieb0e4e24d31780aca1cf291f9d21d49cee181cf2