[A11y] Add battery chart slot selected status in content description.

- Let talkback annouce the slot status before battery slot info.
"Selected": If current slot is selected or after the double-tap action to selected a slot.
"Unselected": If current slot is not the seletectd one or after double-tapping an already selected slot(which will selected ALL instead)

Fix: 386027256
Test: Talkback
Flag: EXEMPT for simple fix
Change-Id: I32a665cdf21620aade8397e5bbe4cb44964b0a39