Support unpacking capitalized property names

Soong config variables may propagate an uppercase name from Make.
Blueprint properties have traditionally been all lowercase, and
using an uppercase property struct field name resulted in a strange
Blueprint property name with the first rune lowercase and the
remaining runes uppercase.

Update the rules for proptools.PropertyNameForField to not lowercase
the first rune if the field name has mulitple runes and is not all
uppercase.

Fixes: 148865218
Test: proptools_test.go
Change-Id: I8de2f65ffb00e5a8ce0aea0caf09f5859315f6b8