Unimplement VrManager#setVrInputMethod()

A system API VrManager#setVrInputMethod(), which was originally
introduced in Android P [1], has hever been used actually.  To avoid
unnecessary maintenance burden in the Android Framework, this CL makes
VrManager#setVrInputMethod() no-op.

For those who want to develop VR Keyboard, they should be able to use
Android P build to see if it actually works and it's actually what
they want.  In the future, if we can have a working prototype, we can
revisit here to see if we want to restore the framework implementation
or not.

Note that with this CL IMEs that have android:isVrOnly="true" will
always be ignored.

There should be no user/developer visible behavior change because this
is about an unused system API.

 [1]: I1db7981b5198e7e203d4578cae7e5b6d20037d0d
      89a6c48a

Bug: 72522822
Fix: 122058241
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest FrameworksCoreTests:InputMethodInfoTest
Test: atest FrameworksServicesTests:InputMethodManagerServiceTests
Change-Id: I5464ff74b92ff4a0d30002d643bd3c89925f0f0c