Add PSKKeyManager to enable the use of TLS-PSK.

TLS-PSK is supported by the underlying TLS/SSL stack (based on
Conscrypt). This CL simply exposes this support in the Android
framework.

TLS-PSK is a set of TLS/SSL cipher suites which rely on a symmetric
pre-shared key (PSK) to secure the TLS/SSL connection and mutually
authenticate its peers. These cipher suites may be a more natural fit
compared to conventional public key based cipher suites in some
scenarios where communication between peers is bootstrapped via a
separate step (for example, a pairing step) and requires both peers
to authenticate each other. In such scenarios a symmetric key (PSK)
can be exchanged during the bootstrapping step, removing the need to
generate and exchange public key pairs and X.509 certificates.

Bug: 15073623
Change-Id: Idf2a773db3021de3d2ad144c5cc63d2415eb8e91