Enable @SoftInputModeFlags annotation in IMMS

This is a preparation to split
com.android.internal.view.InputMethodClient.

When WindowManager.LayoutParams.SoftInputModeFlags annotation was
originally introduced [1], somehow we could not use that annotation in
InputMethodManagerService (IMMS) probably because of some build system
or compiler limitation since the attribute belongs to frameworks.jar
while IMMS belongs to services.jar.

Now it seems that such a limitation no longer exists, there is no
reason to not enable @SoftInputModeFlags annotation in IMMS.

@SoftInputModeFlags is @Retention(SOURCE) anyway. Hence there should
be no behavior difference at run time.

 [1]: I96300b090edce327d0515c740183abe91ded6bac
      22dac1c8

Bug: 118040692
Test: compile
Change-Id: I5fb04a9c63d7123d25f2dbcc7e24b51ca79687ee