[Android] improving "private browsing" icon in PB mode

[kjozwiak]

Description

We should make the PB icon that switches between PB/Normal lighter/white so it's easier to see. It's currently a grey icon against a dark background which makes it hard to spot. Users who don't know the functionality exists might end up missing it.

Steps to reproduce

1. launch the latest released android-core
2. open PB mode and you'll notice the icon in the right corner hard to see/spot

Actual result

Example of issue occurring in PB

Expected result

Icon should appear white so it's easier to see while in PB mode. Users who don't know the switching functionality exists might miss the icon entirely as it's hard to see.

Issue reproduces how often

100% reproducible using the above STR.

Issue happens on

• Current Play Store version? Yes
• Beta build? Yes

Device details

• Install type (ARM, x86): ARM
• Device (Phone, Tablet, Phablet): Samsung Galaxy Tablet A
• Android version: Android 9

Brave version

• Brave-Nightly 1.9.7 CR: 81.0.4044.83

Website problems only

• Does the issue resolve itself when disabling Brave Shields? N/A
• Does the issue resolve itself when disabling Brave Rewards? N/A
• Is the issue reproducible on the latest version of Chrome? N/A

Additional information

CCing @brave/legacy_qa @SergeyZhukovsky @bsclifton

[SergeyZhukovsky]

cc @anthonypkeane @jamesmudgett

[srirambv]

This was reported on tabs long back but never fixed brave/browser-android-tabs#747