Add PackageInstallerSession#getChildSessions (1/n)

Use this method to retrieve child sessions without an additional call
to mStagedSessions.get(id).

Note now getChildSessions[Locked] returns an empty list for a
single-session. The caller should check isMultiPackage() before dealing
with child sessions.

Bug: 167946370
Test: atest StagedInstallTest StagedInstallInternalTest
Change-Id: I186a5f16fd4107994f6426ebca9b16186f7b2cb8