Show IME in default display for some use cases.

- Since IME is a kind of system decoration, for some use cases
  that has not set FLAG_CAN_SHOW_SYSTEM_DECORATIONS flag,
  it means there is no system decorations in display & need to show IME
  in default display.
- For VR 2D display case, we show IME in default display
  when detected VR IME started.
- Add unit tests for verifying above logics in InputMethodManagerServiceTests:
  - testComputeImeDisplayId_defaultDisplayId
  - testComputeImeDisplayId_InvalidDisplayId
  - testComputeImeDisplayId_VrIme
  - testComputeImeDisplayId_noSystemDecorationSupportDisplay
  - testComputeImeDisplayId_withSystemDecorationSupportDisplay

Bug: 114350902
Test: atest FrameworksServicesTests:InputMethodManagerServiceTests
Change-Id: Iedd71e4ddf4983f90b02dd72e471e7fa8e838fbf