Show account selection dialog when a single account exists.

- Recommended fix by security team to prevent auto-importing when there is a single account (see b/388032224#comment4)

FIX: 388032224
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:615666035723ff1828ff24674209012f30082de9)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:5484a98b668d6106cd8c857b5122cafec82a6644)
Merged-In: I83f046c24d45e076aa783e5551b42dcbaacd0ad6
Change-Id: I83f046c24d45e076aa783e5551b42dcbaacd0ad6