Enable printing stack traces for activity starts based on a sysprop

- For non-platform devs who don't have a platform checkout, having the
  ability to enable a stack trace for all processes in the system
  can help further narrow down the origin of the launch.  Having it in
  logcat vs debugging the process can also be helpful if there are other
  adjacent logs to cross-reference with.
- Usage:
    // Enabling
    adb shell setprop persist.wm.debug.start_activity true
    adb reboot
    adb logcat -s "Instrumentation"

    // Disabling
    adb shell setprop persist.wm.debug.start_activity \"\"
    adb reboot

Fixes: 309020623
Test: adb shell setprop persist.wm.debug.start_activity true
Change-Id: I0b2ba15106053daddcaad246ebf97c339298c325