Custom amounts for Brave Rewards not showing in drop down

[bsclifton]

Test plan

See brave/brave-core#910

Description

While implementing brave/brave-core#736 I noticed that custom amounts (ex: values not in the drop-down) are not being added to the drop-down after being set via RewardsServiceImpl::SetContributionAmount.

Steps to Reproduce / test plan for peer review

1. Build from master
2. In the code somewhere, call RewardsServiceImpl::SetContributionAmount and pass a value like 13
3. Visit brave://rewards and notice that the amount is not shown in the drop down

Actual result:

For me it defaulted to 20?

Expected result:

It should show 13

Reproduces how often:

100%

Brave version (brave://version info)

master (brave/brave-core@d76c724)

Reproducible on current release:

n/a, since it involves writing code (discovered while importing from Muon)

[NejcZdovc]

@kjozwiak @rebron @srirambv set this to 0.57 so it will be fixed for muon import. Please adjust it if you think we should not land it there

[NejcZdovc]

adde QA/No as we don't have good steps for QA to use

[GeetaSarvadnya]

Verification passed on

Brave	0.57.12 Chromium: 71.0.3578.75 (Official Build) (64-bit)
Revision	06ef00b5279f93f8e0c1e73acedd49d7dcc09767-refs/branch-heads/3578@{#836}
OS	Windows

• Verified the STR mention in the Adds custom monthly amount to the list brave-core#910

Verified passed with

Brave	0.57.12 Chromium: 71.0.3578.75 (Official Build) (64-bit)
Revision	06ef00b5279f93f8e0c1e73acedd49d7dcc09767-refs/branch-heads/3578@{#836}
OS	Mac OS X

• Verified with STR from Adds custom monthly amount to the list brave-core#910
  

Verification passed on

Brave	0.57.17 Chromium: 71.0.3578.80 (Official Build) (64-bit)
Revision	2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS	Linux