Remove synchronization from getCurrentInputMethodSubtype()

getCurrentInputMethodSubtype() acquires InputManagerService.mMethodMap
within its body. There seems to be no reason for holding
InputMethodManager.mH to call getCurrentInputMethodSubtype(). Holding mH
can cause potential deadlock b/w two threads acquiring mH and mMethodMap
in different orders.

Bug: 31247871
Bug: 31273203
Bug: b.android.com/218423
Change-Id: I20cf2c20f49b1b02c0f7a18257b49d4bcc081b5d