Delay tasks from IMMS constructor to later phase

This is	a preparation CL for Bug 32343335.

This CL basically addresses the concern raised in a previous CL [1]
that added several synchnonized blocks in the constructor of
InputMethodManagerService (IMMS).

After hours of code reading and investigation, it is concluded to be
safe IMMS to behave as if there was no IME available until
SystemService.PHASE_ACTIVITY_MANAGER_READY phase. This allows IMMS to
register callbacks not in the constructor but in the later phase to
addresses the concern that object references to IMMS could be leaked
into different threads before IMMS object is properly constructed.

As far as performance is concerned, basically the amount of tasks
to be done in the main thread remains same, or could be reduced a bit
if some unnecessary callback events were skipped. To confirm this, we
can check the total number of the following performance metrics.
 - StartInputMethodManagerLifecycle
 - StartActivityManagerReadyPhase

 [1]: I9d4d3d7232c984432185c10c13fb726a6158cac8
      8f202f82fd86a3b40bc5e7d62779eddff21096b1

Bug: 32343335
Test: Manual: pre-installed IME on Direct-Boot disabled device
Test: Manual: pre-installed IME on Direct-Boot enabled device
Test: Manual: user-installed IME on Direct-Boot enabled device
Test: Manual: user-installed IME on Direct-Boot disabled device
Test: Manual: user switching scenario on Direct-Boot enabled device
Test: Manual: user switching scenario on Direct-Boot disabled device
Test: Manual: device unlocking scenario on Direct-Boot enabled device
Change-Id: I5b37c450db4b25b3e635b6d634293a34eec8b9d4