Decouple key_repeat_timeout and long_press_timeout

The key repeat timeout was the same as the long press timeout before
Android U. However, due to a long-standing bug, the InputDispatcher
queried the key repeat timeout early in the boot phase before the
long press timeout setting was actually read, which meant that the
default fallback value was being used every time.

Although the key repeat timeout used the long press timeout under the
hood, the behavior from the user's perspective was that the key press
timeout could effectively never be changed.

Code changes in U solved the aforementioned long-standing bug, but the
now-new behavior of key repeats being affected by changes in long
press timeout is unexpected from a user perspective.

We have decided that we want to decouple to two settings altogether,
without having one fall back to the other.

Bug: 310657006
Test: manual, adb shell settings put secure key_repeat_timeout <value>
Change-Id: Ieb7167c3639bad6e7eaa24bdfc657b8ac1de0b8e