Post tasks back to PackageInstaller thread

When PackageManagerService return the verification result back to
PackageInstallerSession, PackageInstallerSession use
PackageManagerService's worker thread to generate the install
parameter.

When the verification tasks are completed, PackageInstaller's worker
thread should execute the following tasks. This patch should make the
benchmark better if PackageManagerService's worker thread is busy.

The benchmark reports that to post runnable is faster than the
original

Test: TID="PackageManagerPerfTests" ; \
           atest \
           ${TID}:android.content.pm.PackageInstallerBenchmark

Bug: 194755410
Change-Id: I0e9b123b999dc3f908819ee40850397a1f3fd9e0