Add tests to verify job scheduling on events

... of rebootless APEX and preload update installation.

Changes made to the actual service:
 * DO_BINARY_MEASUREMENTS_JOB_ID is now a constant, like most of jobs in
   the framework. This makes it easy to use from the host.
 * Replace the local state of sScheduled with a query to JobScheduler,
   which is the source of truth. I've messed up the state during test
   and manual interaction, and this is supposed to make it more
   resilient.

Bug: 265244016
Test: atest BinaryTransparencyHostTest
Test: atest BinaryTransparencyServiceTest
Change-Id: I17872bbded0b5c5c44e6fbd1c669f1eb00f3333e