keystore: Change superclass of UserPresenceUnavailableException.

Also add additional comments to isTrustedUserPresenceRequired().

Test: m -j KeystoreTests && adb install -r
out/target/product/${TARGET_PRODUCT}/data/app/KeystoreTests/KeystoreTests.apk
    adb shell am instrument

Bug: 73392697
Bug: 76462141
Change-Id: Iabcc331adda53e2a5cad5ead6002dfbc29188da2
Signed-off-by: Allen Webb <allenwebb@google.com>