Properly parse meta_key in AccountsDb.

Database entries are stored with "auth_uid_for_type:" prefix.
Remove it properly in findMetaAuthUid.

Bug: 507575871
Test: manual
Flag: EXEMPT BUGFIX
(cherry picked from commit 6a2d8b6bc382fa5dc845650d3a47176efa0ef554)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:9d93fc6dc1a4c4e71c6be4423e002934500fcc4f
Merged-In: I3c028c0cfdbdc44a09fffee3542ba8bc782c1435
Change-Id: I3c028c0cfdbdc44a09fffee3542ba8bc782c1435