Add userspace_reboot_config_prop property type

This property type will be used for read-only userspace reboot related
properties that are used to configure userspace reboot behaviour, e.g.:
* timeout for userspace reboot watchdog;
* timeout for services to terminate;
* timeout for services to shutdown;
* etc.

Since all this configuration is device specific, vendor_init should be
able to set these properties.

Test: build/soong/soong_ui.bash \
  --make-mode \
  TARGET_PRODUCT=full \
  TARGET_BUILD_VARIANT=eng \
  droid \
  dist DIST_DIR=/tmp/buildbot/dist_dirs/aosp-master-linux-full-eng/funwithprops \
  checkbuild
Bug: 135984674
Bug: 147374477

Change-Id: I1f69980aea6020e788d5d2acaf24c0231939907c
4 files changed