Introduce ApexSessionManager and add unit tests for it

This patch just introduces new class that implements same functionality
to the static functions of ApexSession class. The follow-up patch will
migrate apexd codebase to use this new class.

Bug: 288309411
Test: atest ApexTestCases
Change-Id: I4baa848b2e3afa6a23bf8ece98d5210e8fb85946
3 files changed