bug(web): blank key keypress handling leads to unexpected output

While investigating #5301 on an Android 5.0 emulator, I noticed this behavior on the "currency" layer of
sil_euro_latin

Longpressing on the blank keys seem output the key from the neighboring key (fat-finger algorithm).

* Blank key next to to ฿ would output `฿`
* Blank key next to backspace would backspace

Shouldn't the blank keys block fat-fingering output?

## Steps to repro:
1. Load Keyman for Android with the default sil_eruro_latin keyboard
2. Go to "numeric" layer and then "currency" layer
3. Longpress on ฿
4. Verify `฿` is output
5. Longpress on the blank key next to ฿
6. See the longpress above the ฿ key
7. Select the `฿` longpress
6. See `฿` output again
7. I noticed the blank key next to backspace will also backspace a character.


### Configuration
Windows 10
Keyman for Android (master branch)
Keyboard: sil_euro_latin version 1.9.5