fix(snackbar): Doesn't close while other element is focused

[piotr-cz]

Snackbar doesn't close when any element on page receives focus event while snackbar is opened and snackbar has empty action text.

Fixes #1918

Steps to reproduce:

1. Go to https://material-components-web.appspot.com/snackbar.html
2. Delete Action Text value
3. Click on Show
4. Press Shift-Tab to focus some element on the page (like Action Text input element)

Snackbar should close, but it doesn't.

[piotr-cz]

One test failed (TEST_SUITE=unit): but it looks like tests configuration problem: The SUID sandbox helper binary was found, but is not configured correctly

[codecov-io]

Codecov Report

Merging #2183 into master will decrease coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2183      +/-   ##
==========================================
- Coverage   98.36%   98.24%   -0.12%     
==========================================
  Files         120      120              
  Lines        5125     5129       +4     
  Branches      637      638       +1     
==========================================
- Hits         5041     5039       -2     
- Misses         84       90       +6

Impacted Files	Coverage Δ
packages/mdc-snackbar/foundation.js	96.02% <100%> (-3.98%)	⬇️
packages/mdc-snackbar/index.js	100% <100%> (ø)	⬆️

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[williamernest]

LGTM. This component will be rebuilt in the future, but this PR fixes the issue for now. Thanks for submitting!