Prevent a deadlock in #abandon

See b/159568595#comment1 for analysis.

1. rename cleanStageDir to cleanStageDirNotLocked to show it must be
   called without mLock held.
2. rename getChildSessions to getChildSessionsNotLocked to show it must
   be called without mLock held.
3. add cleanStageDir(List<PackageInstallerSession>) which is callable
   when mLock is held.

Bug: 159568595
Test: atest StagedInstallTest
Change-Id: I512ea07a4f8750431b306fa6e3c47a600a50b64f