Declare MY_PID and MY_UID in WMS

- Reduce unnecessary invocation.
- Reduce references to am package.
- Though MY_UID can be replaced with SYSTEM_UID, still keep
  it in some places for readability (e.g. indicate owner).

Bug: 163976519
Test: CtsWindowManagerDeviceTestCases

Change-Id: I0cfb7e552bea7eb13140e7e1db17a08f257a86ac