Some code cleanup (2/n)

1. Remove cleanStageDirNotLocked and getChildSessionsNotLocked since
   now we can safely retrieve child sessions within mLock.
2. Remove the null-check in #cleanStageDir since we never store
   null in mChildSessions.

Bug: 162386287
Test: atest StagedInstallTest
Change-Id: Ieed57ea6526507c0961c3dcd40b50d4fe3d4f02b