Use IMMS' worker thread for IMMS.MyPackageMonitor

This CL attempts to mitigate performance regressions that could have
been caused by my previous CLs [1][2], which might have increased
tasks executed on a shared background thread in system_server.

By default

  com.android.internal.content.PackageMonitor

runs on

  com.android.internal.os.BackgroundThread

unless a looper is specified explicitly. What I suspect is that with
my previous CLs IMMS.MyPackageMonitor started taking more time on that
shared background thread, which results in slowing down other
components in the system_server process.

To mitigate the above scenarios, let's use IMMS' own worker thread
to handle IMMS.MyPackageMonitor callbacks.x

 [1]: I07e32739f486d960c9dd22476120fa35bf1899e4
      0c5feb22
 [2]: I10aa547f0de607ef3c0ba26764dac0585c40c843
      2501e749

Bug: 327861441
Test: presubmit
Change-Id: I1ed05d753ca72326aeb5eaf7b402f0d280ed5dc8