Remove unnecessary internal lock

Previously, InputMethodSubtypeSwitchingController has relied on
its own internal lock for #getNextInputMethod and
class has to be invalidated whenever
InputMethodManagerService#mMethodMap is updated, any method of
InputMethodSubtypeSwitchingController should be called under
the global lock of InputMethodManagerService#mMethodMap.

As a consequence, we can conclude that
InputMethodSubtypeSwitchingController does not need its own
internal lock.

This CL also adds additional synchronization blocks into
the constructor of InputMethodManagerService to address the
existing inconsistency that methods with *Locked suffix are
called without the lock actually.

BUG: 7043015
Change-Id: I9d4d3d7232c984432185c10c13fb726a6158cac8