Fixing bugs related to pausing of private space.

Added variables to store state of the initial stack and the last
selected user in BaseActivity, which is used in DirectoryFragment to
reset the stack if the last selected user befor pausing of the private
space happened to be private user itself.

Bug: 325010006
Bug: 323589103
Bug: 324999992
Test: atest unit test
Change-Id: I398f9f4d2ac0bd24f9f457fdff4b5a965cd6f67c