Tell the reason why IMM called startInput().

As a preparation to fix Bug 25373872, this CL introduces an additional
int parameter into the following two methods
  - IInputMethodManager.startInput()
  - IInputMethodManager.windowGainedFocus()
so that IMMS can know why IMM needs to start input.  Currently the
"startInputReason" parameter is used only for debug message only when
the OS is rebuilt with flipping IMMS#DEBUG to true.  Basically this
should have no impact in production builds except for a tiny overhead
of having one int parameter in some internal IPC calls.

Note that since 7663d80f [1] basically
IMMS#windowGainedFocus() has been a superset of IMMS#startInput().
Hence we should pass to "startInputReason" parameter to
IMMS#windowGainedFocus() as well as IMMS#startInput().

  [1]: Icb58bef75ef4bf9979f3e2ba88cea20db2e2c3fb

Bug: 25373872
Change-Id: Ia1fe120af7d71495c5f3a4fc0ec6390efb8240ca