Fix TrustManagerServiceTest failures due to PackageMonitor changes

Some test cases in TrustManagerServiceTest are failing on internal main
because http://ag/23669695 changed how PackageMonitor works.  Sending an
ACTION_PACKAGE_CHANGED intent no longer causes
PackageMonitor#onPackageChanged to be called.  This CL fixes the test
failures by making TrustManagerServiceTest call the onPackageChanged
method of TrustManagerService's PackageMonitor directly.

Bug: 29385425
Test: atest TrustManagerServiceTest # on both AOSP and internal main
Change-Id: I0b4708e4223452430ff10e422a71b223656a3360