Remove old support lib dependencies

[rmeissner]

Changes proposed in this pull request:

• Replace android support lib with androidx
• Return created snackbar in snackbar extensions (allows to hide the snackbar)

@gnosis/mobile-devs

[codecov-io]

Codecov Report

Merging #36 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #36   +/-   ##
=========================================
  Coverage     62.33%   62.33%           
  Complexity      152      152           
=========================================
  Files            54       54           
  Lines          1346     1346           
  Branches        223      223           
=========================================
  Hits            839      839           
  Misses          406      406           
  Partials        101      101

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