Allow different explicit health and package expiry timeouts

We have always evaluated the explicit health check results on package
expiry. Since I29e2d619a5296716c29893ab3aa2f35f69bfb4d7 we now receive
explicit health check timeouts from ExtServices. This cl doesn't yet
use the timeout but it treats explicit health check timeouts as
different events from package expiry. This is in preparation to use
the timeouts from the cl mentioned above.

Improved readability: Logging, comments, variable and function names

Bug: 120598832
Test: atest PackageWatchdogTest
Change-Id: I8030dae1fef5b8fee42095c1eaf16861cc33ac59