Add InputMethodSubtype#isSuitableForPhysicalKeyboardLayoutMapping()

This is a preparation for implementing Subtype-keyed physical keyboard
layout switching.

This CL introduces an @hide method

  InputMethodSubtype#isSuitableForPhysicalKeyboardLayoutMapping()

so that later CLs can be written on the same rule about what kind of
InputMethodSubtype is valid for physical keyboard layout swithcing.

Here is the current rule but it's subject to change.

 * hashCode() != 0 (SUBTYPE_ID_NONE)
 * getMode() must be "keyboard"
 * not isAuxiliary()
 * getCanonicalizedLanguageTag() returns a valid (non "und") value.

See test cases to see how it works.

Bug: 252816846
Test: atest FrameworksCoreTests:InputMethodSubtypeTest
Change-Id: Ifc0247041a43ef64f8a76a23832da2ee058c6958