Clean up key handling in adb.

This includes the locking we need to be able to re-load the keys at runtime.

We should rename "adb_auth_client.cpp" to "adb_auth_adbd.cpp" or
"adbd_auth.cpp" in a later change.

Change-Id: I9e1d5b6b7d0497d6f6e5d9c4fb660118cdff05a8
Test: "adb devices" works against a non-AOSP device with $ADB_VENDOR_KEYS set, says "unauthorized" without.
Bug: http://b/29273531