Enable clock only when supported by product

In 9748086f, a detection for clock drifting was implemented. It relies on
2 files that must be provided by the product
(/data/system/time/last_time_change and
/data/system/time/time_is_accurate). Generally, these files are not
available which triggers an SELinux denial error message.

Introduce a new property ("persist.sys.bootanim.clock.enabled") which
can be set by a product to enable this feature (default is false). For
future reference, it is not sufficient to only enable this property, the
product must also provide the files described above (see
BootAnimation::updateIsTimeAccurate).

Bug: 62954877
Test: lunch r11-userdebug && m
Change-Id: Ia27e69547e3d898925029dc0c2be4ad717891c57