Promotions are fetched too often

[NejcZdovc]

We should have some cacheing in this area, so that we don't pull every time that you open a panel for example. We should store last fetch in preferences (last_fetch.promotion) and then when we get request for promotion fetch we check that preference and if's not older then 10min just return what we have in db. We also need to allow for this check to be removed if you pass is_testing set

[LaurenWags]

Verified passed with

Brave | 1.12.94 Chromium: 84.0.4147.89 (Official Build) dev (64-bit)
-- | --
Revision | 19abfe7bcba9318a0b2a6bc6634a67fc834aa592-refs/branch-heads/4147@{#852}
OS | macOS Version 10.14.6 (Build 18G3020)

• Verified test plan from Retrieve promotions from db when fetched recently brave-core#5760
• Verified on staging env with the following flags --args --enable-logging=stderr --vmodule="*/bat-native-ledger/*"=6,"*/brave_rewards/*"=6,"*/bat-native-ads/*"=6,"*/bat-native-confirmations/*"=6,"*/brave_ads/*"=6 --brave-ads-staging --rewards=staging=true --v=1
• Confirmed after claiming the grant, there were no lines in the terminal like [3346:13827:0720/153245.371484:VERBOSE1:network_delegate.cc(32)] NetworkDelegate::NotifyBeforeURLRequest: https://grant.rewards.bravesoftware.com/v1/promotions?migrate=true&paymentId=2e39e07b-74ea-4f54-bd62-2828bb09d142&platform=osx until after >10 minutes had passed, then opening the panel triggered another fetch.

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Verification passed on

Brave | 1.12.99 Chromium: 84.0.4147.89 (Official Build) dev (64-bit)
-- | --
Revision | 19abfe7bcba9318a0b2a6bc6634a67fc834aa592-refs/branch-heads/4147@{#852}
OS | Windows 10 OS Version 1903 (Build 18362.959)

• Verified test plan from Retrieve promotions from db when fetched recently brave-core#5760
• Verified on staging env with the following flags --args --enable-logging=stderr --vmodule="*/bat-native-ledger/*"=6,"*/brave_rewards/*"=6,"*/bat-native-ads/*"=6,"*/bat-native-confirmations/*"=6,"*/brave_ads/*"=6 --brave-ads-staging --rewards=staging=true --v=1

[18352:5112:0723/202536.446:VERBOSE1:network_delegate.cc(32)] NetworkDelegate::NotifyBeforeURLRequest: https://grant.rewards.bravesoftware.com/v1/promotions?migrate=true&paymentId=075fe099-961c-4435-9fb9-dc9910d65fb9&platform=windows

• Confirmed after claiming the grant, there were no lines in the terminal like

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Verification passed on

Brave	1.12.102 Chromium: 84.0.4147.89 (Official Build) dev (64-bit)
Revision	19abfe7bcba9318a0b2a6bc6634a67fc834aa592-refs/branch-heads/4147@{#852}
OS	Ubuntu 18.04 LTS

• Verified test plan from Retrieve promotions from db when fetched recently brave-core#5760
• Verified on staging env with the following flags --args --enable-logging=stderr --vmodule="*/bat-native-ledger/*"=6,"*/brave_rewards/*"=6,"*/bat-native-ads/*"=6,"*/bat-native-confirmations/*"=6,"*/brave_ads/*"=6 --brave-ads-staging --rewards=staging=true --v=1
• Confirmed after claiming the grant, there were no lines in the terminal like [4542:4550:0727/202256.933719:VERBOSE1:network_delegate.cc(32)] NetworkDelegate::NotifyBeforeURLRequest: https://grant.rewards.bravesoftware.com/v1/promotions?migrate=true&paymentId=90bfb7db-0f74-4752-bbad-326b2aa1dcc7&platform=linux until after >10 minutes had passed, then opening the panel triggered another fetch.

[srirambv]

Verification passed on OnePlus 6T with Android 10 running 1.12.112 x64 build

• Verified steps from Retrieve promotions from db when fetched recently brave-core#5760
• Verified log shows a request for promotion with the platform details
  
• Verified no log after claiming the grant
• Verified after ~10 mins opening the panel again and checking logs shows a second request sent to the server