Replace function type in interface w/ reflectable version

This issue was due to passing a builtin kotlin type (in this case
Function<>) across the api boundary. This caused there to be two
versions of the target type, which could not be reconciled at
runtime.

Unlike previous similar bugs, neither SystemUI nor WPP crashed due
to this, but instead both successfully fell back to using the default
clock in non-debug builds.

Bug: 400469538
Flag: NONE bugfix
Test: Manually checked operation of custom clocks
Change-Id: I0d7ac683130c360a672f79ea7dd753669f4b97dc