[pm] move RemoteCallbackList.broadcast to handler thread

Fixing a system server crash because RemoteCallbackList.broadcast cannot
be called by multiple threads simultaneously. Move it to the handler
thread to avoid that.

BUG: 289240865
Test: builds
Test: atest PackageMonitorCallbackHelperTest
Test: atest CtsAppTestCases
Test: atest CtsDevicePolicyTestCases

Change-Id: I7d6b2c282f031953862fe9cac8c3bb7449ac50b0