Bootstrapping bar needs a little TLC on Android

The bootstrapping bar on Android needs a little TLC to match the designs that were produced in #41188 (closed), namely:

• The terminus of the progress bar (i.e. the colorful portion) should be fully rounded on its right-most end.
• The track behind the progress bar should be a flat purple color.
• The progress bar should appear in motion even when bootstrapping is static, by rotating the gradient, as is done on desktop.