Don't hold lock calling into PackageMgr

There's a subtle deadlock; when dumping state, we obtain the package manager
lock before trying to obtain the package installer session lock. Meanwhile,
creating a new session obtains the locks in the reverse -- the package
installer takes the package installer session lock and then tries to obtain
the package manager lock.

Here, we avoid holding the package installer session lock before we call
into the package manager. Alternatively, we could consider not holding the
package manager lock when dumping package installer state. But, given the
scope of the dumping logic, that's a bigger change with other, unknown
side effects.

Change-Id: I11f08484cd335bb7ad3bc557808eb48d14bd29cf
Fixes: 30089638