Fix translucent navbar for Android 10

[svank]

When the status bar is not hidden (i.e. StfalconImageViewer.Builder().withHiddenStatusBar(false)), past behavior was for the
navigation bar to be translucent. On Android 10 (SDK 29), it is opaque
black. See screenshot below, Android 9 on left and Android 10 on right

This patch restores and improves upon past behavior for Android 10 by
ensuring a fully transparent nav bar when gesture controls are enabled
(and using the system-provided translucent scrim when 3-button navigation
is enabled). See screenshot below, Android 9 on left,
Android 10 center and right with three-button and gesture navigation.

I'm not 100% sure I've done this in the optimal way, but here's what I've found:

To achieve this we need to switch from using
android:windowTranslucentNavigation to using android:navigationBarColor.
On Android 10, it appears android:windowDrawsSystemBarBackgrounds
must be set for android:navigationBarColor to have any affect.
(I wonder if this is related to using an AlertDialog.)
android:windowDrawsSystemBarBackgrounds is not supported in SDK < 21,
whereas this library supports SDK >= 19, so I've created a separate
styles.xml file for SDK >= 29.

Setting systemUiVisibility flags appears to be necessary to get
the transparent nav bar effect. It seems that setting
android:windowTranslucentNavigation was providing the same effect
as setting these flags, but now it needs to be done manually for
SDK >= 29 as android:windowTranslucentNavigation is not set.

[benjictln]

Thanks for this PR ! It solved my issue, I copy pasted the code you added in styles.xml (in v29) and it worked for me.

[benjictln]

actually, it wasn't translucent for me, I actually added this code on my style.xml for anyone interested:

    <style name="ImageViewerDialog.DefaultBaseTheme" parent="android:Theme.Translucent.NoTitleBar">
        <item name="android:windowTranslucentStatus">true</item>
        <item name="android:windowTranslucentNavigation">true</item>
        <item name="android:navigationBarColor">@color/alwaysBlack</item>
    </style>

<!--  inspired by https://github.com/stfalcon-studio/StfalconImageViewer/pull/38, override the property of StfalconImageViewer lib -->

    <style name="ImageViewerDialog.Default" parent="ImageViewerDialog.DefaultBaseTheme">
        <item name="android:windowDrawsSystemBarBackgrounds">true</item>
        <item name="android:windowTranslucentNavigation">true</item>
        <item name="android:navigationBarColor">@color/alwaysBlack</item>
    </style>