Simpler handlers in selendroid-standalone

Summary:
- Make logging of requests in selendroid-standalone consistent by doing it in the `BaseSelendroidServerHandler` super class.
- Rename `RequestRedirectHandler` to `ProxyToDeviceHandler`. That class has nothing to do with redirection (as in HTTP redirect) so the current name is confusing.
- Random simplification of some messages and cleanup of typos (e.g. fix the spelling of "occurred").
- Rename "session key" to "session id". The [[http://www.w3.org/TR/webdriver/|WebDriver spec]] talks about ids, not keys.

Test Plan:
- `mvn clean install`
- Ran one of the Facebook end-to-end tests.

Reviewers: ainsej, kumar.rangarajan, juyuan.yang.9, asm

Reviewed By: asm

Differential Revision: https://reviews.facebook.net/D29697