Remove limit of system property name length

This change introduces new __system_property_read_callback
method to use in place of deprecated __system_property_read
__system_property_set() and get() should just work but now
do not have limit on system property names.

Bug: http://b/33926793
Test: boot device, run adb shell propget
Test: boot device with old version of init (protocol v1)
Test: run bionic-unit-tests --gtest_filter=prop*
Change-Id: I619fb5a7e27a272aac30011579665f6160888bc7