Allow keycode lookup without KEYCODE_ prefix

Currently, KeyEvent.keyCodeFromString(String name) requires the string
to either start with "KEYCODE_", or be directly convertible to an int.
However, the string representation of every keycode starts with
"KEYCODE_", so this requirement is redundant. Relax this requirement to
alllow both of the following usages:
1) keyCodeFromString("KEYCODE_BUTTON_A")
2) keyCodeFromString("BUTTON_A")

Currently, only 1) is supported.

The other usage,
3) keyCodeFromString("29")
is unchanged.

The input is no longer case-sensitive.
Improved the example of usage in the documentation: the input
"1001" suggests that the string must contain binary representation for
usage 3), while in fact it is supposed to be a base 10 number.

Test: atest cts.KeyEventTest#testKeyCodeFromString
Bug: 36069459

Change-Id: I54d7f9d1270748854143cc9d1e8af48c9ec0cd0f