WA-97: Add possibility for hw encryption

[rmeissner]

Closes WA-97.

Changes proposed in this pull request:

• KeyStorage interface to allow additional functionality/security when storing the key
• AndroidKeyStorage that uses the AndroidKeyStore to encrypt a key

@gnosis/mobile-devs

[codecov-io]

Codecov Report

Merging #26 into master will decrease coverage by 2.06%. The diff coverage is 10.86%.

@@             Coverage Diff              @@
##             master      #26      +/-   ##
============================================
- Coverage     66.41%   64.34%   -2.07%     
- Complexity      146      147       +1     
============================================
  Files            49       50       +1     
  Lines          1197     1237      +40     
  Branches        185      192       +7     
============================================
+ Hits            795      796       +1     
- Misses          314      350      +36     
- Partials         88       91       +3

Impacted Files	Coverage Δ	Complexity Δ
...gnosis/svalinn/security/impls/AndroidKeyStorage.kt	0% <0%> (ø)	0 <0> (?)
...va/pm/gnosis/svalinn/security/EncryptionManager.kt	53.84% <100%> (ø)	0 <0> (ø)	:arrow_down:
...sis/svalinn/security/impls/AesEncryptionManager.kt	73.94% <33.33%> (-3.73%)	24 <1> (+1)

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