Rename DPM.isDefaultInputMethodSetByOwner()

Settings.Secure.DEFAULT_INPUT_METHOD is a misnomer. It does not really
record a permanent default of any sort - it just indicates the currently
chosen IME. Thus, isDefaultInputMethodSetByOwner() should more
appropriately be called isCurrentInputMethodSetByOwner().

Furthermore, it turns out that setting a different IME for a user and
the user's work profile is unsupported. Thus, it is sufficient for the
intended use case to just retrieve the calling user's default IME.
There is no need for a |user| parameter.

Bug: 32692748
Test: unit tests (see DevicePolicyManagerTest.java for invocation)
Test: cts-tradefed run cts-dev --module CtsDevicePolicyManagerTestCases

Change-Id: Ia0846d38a1361042429dae7430a8b055575ef2e0