[Android] Implement fractional tipping amounts (e.g. 1.25 BAT) on Android

[stephendonner]

Description

Implement fractional tipping amounts (e.g. 1.25 BAT) on Android (found while testing #15050)

Steps to reproduce

1. clean profile
2. launch Brave
3. go to About > Settings > Developer Options > QA Preferences
4. enter the QA code
5. toggle Use Rewards staging server to On
6. relaunch Brave
7. enabled Rewards
8. load 3zsistemi.si
9. tap on the BAT icon in the URL bar
10. take note of the default tipping values

Actual result

[1.000 BAT ] [5.000 BAT] [10.000 BAT]

Expected result

To match desktop, they should be -- for 3zsistemi.si at this time, on staging -- 1.25 BAT, for example

[1.25 BAT ] [5 BAT] [10.5 BAT]

Issue reproduces how often

Version/Channel Information:

• Can you reproduce this issue with the current Play Store version? yes
• Can you reproduce this issue with the current Play Store Beta version? yes
• Can you reproduce this issue with the current Play Store Nightly version? yes

Device details

• Install type (ARM, x86): arm64
• Device type (Phone, Tablet, Phablet): Pixel XL
• Android version: 9.0

Brave version

Brave 1.24.82, Chromium 90.0.4430.93

/cc @deeppandya @zenparsing

[Miyayes]

I believe this is because the values are currently hardcoded on Android and aren't fetched. Will probably be solved with #17070

[Miyayes]

Going to close this in favor of #17070, since this is a subset of #17070.