Find wall clock RTC through sysfs

Devices may have multiple RTCs.  By default the kernel uses rtc0 to
store the system time, but devices may override this (or even specify
that none of them should be used for system time).

Userspace can indirectly find the designated RTC through sysfs.  During
AlarmManagerService initialization, enumerate through all rtc class
devices to locate the device with attribute hctosys=1.

This is only done on devices without /dev/alarm, which has its own
in-kernel mechanism to pick the RTC.

Change-Id: Ife2b342c3590133ed316ddaf1799cbc1bfa6e6d9
Signed-off-by: Greg Hackmann <ghackmann@google.com>