Improve captalization in InputMethodSubtype#getDisplayName().

The goal of this CL is to enable
InputMethodSubtype#getLocaleDisplayName() to return more natural locale
display name in terms of capitalization rules.

The key idea here is to use LocaleDisplayNames#localeDisplayName() with
an appropriate DisplayContext.CAPITALIZATION_FOR_* parameter rather than
relying on Locale#getDisplayName().

Bug: 22845728
Change-Id: If105082ce703db7a86738455db7e9fb37f3c6fe8