Stop talkback from saying "false" when focused on Open in browser button

The imageview in the button had a content description which was being
read out unintentionally, so by marking the image as not important for
a11y, it will now be ignored, and just the button text will be read out

Fix: 398128081
Test: Open app handle menu and focus with talkback
Flag: EXEMPT bugfix
Change-Id: Idb4967a814cbdb19c058459ed1b4cf298dac7a48