Fix job panel display issues in RTL

- The popup displays in the wrong location, so use gravity to position
  it instead of an x offset.
- The signal title message text view has the text always pushed to the
  left, so change the alignment.

Bug: 412566817
Test: atest -c 'DocumentsUIGoogleTests:com.android.documentsui.JobPanelUiTest'
Flag: com.android.documentsui.flags.visual_signals_ro
Change-Id: Ie54f75dc2331736009443ba84da2066ace0fefa4