[Android] Add drop shadow around Rewards dropdown panel

[Miyayes]

Description

The Rewards dropdown panel melds into the underlying webpage on Android because there is no dropshadow separating it from the background:

It should be more like desktop, where there's a dropshadow that sets it apart:

[Miyayes]

Android 6/7 APIs different, so we need to make sure it works on all the devices.

[stephendonner]

Verified PASSED on Google Pixel XL (arm64) running Android 9:

Brave	1.52.102 Chromium: 113.0.5672.126 (Official Build) beta (32-bit)
Revision	c541687b21a73452ab403e2dced7033ddc97ee9d-refs/branch-heads/5672@{#1202}
OS	Android 9; Build/PQ2A.190305.002; 28; REL

Steps:

1. installed 1.52.102
2. launched Brave
3. enabled Rewards staging via QA Debug menu
4. relaunched Brave
5. logged into Uphold's staging sandbox
6. loaded laurenwags.github.io
7. tapped on the Rewards icon
8. tapped to enable Rewards
9. tapped again to invoke the dropdown Rewards panel

Confirmed the dropshadow is now present around the panel

1.52.102	1.51.114

[hffvld]

Verified on Nexus 5X, Galaxy Tab S8 and Google Pixel 7 using version(s):

Device/OS: 
- Google Nexus 5X [Android 6.0; Android 7.1.1; Android 8.1]
- SM-X700 Galaxy Tab S8 [gts8wifixx-user 13 TP1A.220624.014 release-keys]
- Google Pixel 7 [panther_beta-user 13 UPB2.230407.014 release-keys]
Brave build: 1.52.102
Chromium: 113.0.5672.126 (Official Build) beta (64-bit)
Revision: c541687b21a73452ab403e2dced7033ddc97ee9d-refs/branch-heads/5672@{#1202}

Filed follow-up issue #30495

---

STEPS:

1. Install build 1.52.102
2. Launch Brave > Hamburger Menu > Settings
3. Enable Rewards Staging via QA Debug menu > Relaunch Brave
4. Tap on the Rewards icon > Enable Rewards
5. Connect account > Sign in to Uphold or Gemini account
6. Open wikipedia.com, for example
7. Tap Rewards icon to invoke the dropdown Rewards panel

ACTUAL RESULTS:

• Verified that drop shadow is shown for the Rewards panel in Light and Dark Modes

Android 6 - Not supported

1

Android 7 - PASS

Light Mode

Android 8 - PASS

Light Mode	Dark Mode

Android 13 - PASS

Light Mode	Dark Mode

Android 14 - PASS

Light Mode	Dark Mode