setExplicitHealthCheckEnabled() needs to set mSyncRequired (6/n)

We need to call mHealthCheckController.syncRequests() whenever
ExplicitHealthCheckEnabled changes. However, syncRequests() fails
to figure this out because it detects changes in
mRequestedHealthCheckPackages only.

Bug: 178675924
Test: atest PackageWatchdogTest
Change-Id: Ic752c7f35c06a0ce770e07d1a186dd9da88b4f1e