Fix talkback speech for storage size unit 'byte'

Formatter#formatBytes returns file size in units B/KB/MB/GB.
Talkback cannot handle unit size B. Use TtsSpan to return a CharSequence
for Talkback to announce this correctly in the Storage Settings page.

BUG: 317747133
Test: manual - open Storage settings page with Talkback feature on
Flag: EXEMPT bugfix
Change-Id: I65d0abb5a9ea358c116f84a85f0e42187998fd17