ANDROID: timekeeping: Export the boot clock in snapshots

The boot clock is interesting for tracing purpose as it doesn't stop on
device suspend. Exporting it intends to let the nVHE hypervisor for the
arm64 architecture to "replicate" that clock and allow event
synchronization with the host. Replicating implies to know the current
slope.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Bug: 278749606
Change-Id: Iefb6ffc433dac82297401f9acdff9758cc1b6a89