Fix capitalization of "false" in flatpakref keyfiles

g_key_file_get_boolean() only accepts lower case "true" and "false", so
correct some instances of "False" in key files. This makes no functional
difference as long as the error pointer isn't checked since
g_key_file_get_boolean() also returns false when it can't parse the
value as a boolean. But it still seems good to be correct, and someone
could interpret the existence of "IsRuntime=False" as implying that
"IsRuntime=True" will work, which it doesn't.

(cherry picked from commit b0981912520de706f1e20993f3486ce83cd5787a)