DO NOT MERGE Check fingerprint client against top activity in auth callback

Due to a race condition with activity task stack broadcasts, it's
currently possible for fingerprint authentication to succeed for a
non-top activity. This means, for example, that a malicious overlay
could be drawn in order to mislead the user about what they are
authenticating for.

This commit addresses the issue by adding a check to the fingerprint
authentication client interface that ensures the authenticating
activity is on top at the time of authentication. Otherwise, the
pending authentication will fail, as if an incorrect biometric
been presented.

Test: Follow steps from b/159249069:
1. Install com.pro100svitlo.fingerprintauthdemo from the Play store.
2. Install the PoC attack app from b/159249069.
3. Start the PoC attack app and press the "Launch PoC attack" button.
4. Use fingerprint to authenticate while the overlay is showing.

Before: Authentication succeeds, and a new activity is launched.
After: Authentication fails, and no new activity is launched.

Bug: 159249069
Change-Id: Ic482cbfdd810e9c8b60a5093fd8d615ee320a6a6
Merged-In: I0707c3f55eaf2a69c6625a3ceb3b5626b3676b26
Merged-In: If5cdf8ffaf3aa7d8a1ac81272e3bfb2cc7cdddf1
Merged-In: Iee6af379515385777984da55048c1efd9339ed88
Merged-In: I9b242a9fee0acbfb430875061e2d809c00fe4b97
Merged-In: I1241a12eafa0bdbac59a8ddd4cf6a0637d467b19
Merged-In: Ie5a0f8c3e9b92d348a78678a6ed192d440c45ffc
Merged-In: I289d67e5c7055ed60f7a96725c523d07cd047b23