Closes #354: Update asset overview screen

[elgatovital]

Update asset overview screen #354

Changes proposed in this pull request:

• Remove send/receive buttons
• Tapping the identicon or the address brings up the receive screen.
• Tapping "Add token" brings up the manage tokens screen.
• Tapping an asset will start the send transaction flow.
• All amounts are formatted as short amount

see https://github.com/gnosis/safe/blob/master/specs/assets_overview/02_user_interface.rst

@gnosis/mobile-devs

[rmeissner]

Only checked the code, did not check how the layout looks

[codecov-io]

Codecov Report

Merging #360 into master will decrease coverage by 0.02%.
The diff coverage is 17.39%.

@@             Coverage Diff             @@
##             master    #360      +/-   ##
===========================================
- Coverage     35.33%   35.3%   -0.03%     
  Complexity      446     446              
===========================================
  Files           239     240       +1     
  Lines          7084    7090       +6     
  Branches       1025    1027       +2     
===========================================
  Hits           2503    2503              
- Misses         4354    4358       +4     
- Partials        227     229       +2

Impacted Files	Coverage Δ	Complexity Δ
...imdall/ui/tokens/balances/TokenBalancesFragment.kt	0% <ø> (ø)	0 <0> (ø)	⬇️
...m/gnosis/heimdall/ui/safe/main/SafeMainActivity.kt	1.61% <ø> (+0.01%)	0 <0> (ø)	⬇️
...is/heimdall/ui/safe/details/SafeDetailsContract.kt	100% <ø> (ø)	1 <0> (ø)	⬇️
...s/heimdall/ui/safe/details/SafeDetailsViewModel.kt	58.33% <0%> (-29.17%)	4 <0> (ø)
...eimdall/ui/tokens/balances/TokenBalancesAdapter.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...is/heimdall/ui/safe/details/SafeDetailsFragment.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...is/heimdall/data/repositories/models/ERC20Token.kt	70.58% <0%> (-5.89%)	9 <1> (ø)
...n/java/pm/gnosis/heimdall/helpers/AddressHelper.kt	100% <100%> (ø)	3 <0> (ø)	⬇️
.../gnosis/heimdall/ui/safe/main/SafeMainViewModel.kt	93.33% <100%> (ø)	15 <1> (ø)	⬇️
...ctions/view/viewholders/AssetTransferViewHolder.kt	85.18% <100%> (ø)	15 <0> (ø)	⬇️
... and 2 more

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