[Wi-Fi] [a11y] In Wi-Fi data usage page, Talkback should also announce the...

[Wi-Fi] [a11y] In Wi-Fi data usage page, Talkback should also announce the Progress bar percentage instead of "Progress bar" only.

Add conent description for talkback to announce, if the progress bar is 50%, then before modification is "Progress bar", and after modification is "50% Progress bar".

Bug: 146811265
Test: Add following unit test case to test if content description is correct or not:
      createPref_progressBarShouldSetPercentageContentDescription().

Change-Id: I71483df8e12f08c4c8c96ab3964666bf97872a83